WASHINGTON — President Trump’s erratic handling of the coronavirus outbreak, the worsening economy and a cascade of ominous public and private polling have Republicans increasingly nervous that they are at risk of losing the presidency and the Senate if Mr. Trump does not put the nation on a radically improved course.

The scale of the G.O.P.’s challenge has crystallized in the last week. With 26 million Americans now having filed for unemployment benefits, Mr. Trump’s standing in states that he carried in 2016 looks increasingly wobbly: New surveys show him trailing significantly in battleground states like Michigan and Pennsylvania, and he is even narrowly behind in must-win Florida.

Democrats raised substantially more money than Republicans did in the first quarter in the most pivotal congressional races, according to recent campaign finance reports. And while Mr. Trump is well ahead in money compared with the presumptive Democratic nominee, Joseph R. Biden Jr., Democratic donors are only beginning to focus on the general election, and several super PACs plan to spend heavily on behalf of him and the party.

Perhaps most significantly, Mr. Trump’s single best advantage as an incumbent — his access to the bully pulpit — has effectively become a platform for self-sabotage.

His daily news briefings on the coronavirus outbreak are inflicting grave damage on his political standing, Republicans believe, and his recent remarks about combating the virus with sunlight and disinfectant were a breaking point for a number of senior party officials.

On Friday evening, Mr. Trump conducted only a short briefing and took no questions, a format that a senior administration official said was being discussed as the best option for the president going forward.

Glen Bolger, a longtime Republican pollster, said the landscape for his party had become far grimmer compared with the pre-virus plan to run almost singularly around the country’s prosperity.

“With the economy in free-fall, Republicans face a very challenging environment and it’s a total shift from where we were a few months ago,” Mr. Bolger said. “Democrats are angry, and now we have the foundation of the campaign yanked out from underneath us.”

Mr. Trump’s advisers and allies have often blamed external events for his most self-destructive acts, such as his repeated outbursts during the two-year investigation into his campaign’s dealings with Russia. Now, there is no such explanation — and, so far, there have been exceedingly few successful interventions regarding Mr. Trump’s behavior at the podium.

Representative Tom Cole, Republican of Oklahoma, said the president had to change his tone and offer more than a campaign of grievance.

“You got to have some hope to sell people,” Mr. Cole said. “But Trump usually sells anger, division and ‘we’re the victim.’”

There are still more than six months until the election, and many Republicans are hoping that the dynamics of the race will shift once Mr. Biden is thrust back into the campaign spotlight. At that point, they believe, the race will not simply be the up-or-down referendum on the president it is now, and Mr. Trump will be able to more effectively sell himself as the person to rebuild the economy.

“We built the greatest economy in the world; I’ll do it a second time,” Mr. Trump said earlier this month, road-testing a theme he will deploy in the coming weeks.

Still, a recent wave of polling has fueled Republican anxieties, as Mr. Biden leads in virtually every competitive state.

The surveys also showed Republican senators in Arizona, Colorado, North Carolina and Maine trailing or locked in a dead heat with potential Democratic rivals — in part because their fate is linked to Mr. Trump’s job performance. If incumbents in those states lose, and Republicans pick up only the Senate seat in Alabama, Democrats would take control of the chamber should Mr. Biden win the presidency.

“He’s got to run very close for us to keep the Senate,” Charles R. Black Jr., a veteran Republican consultant, said of Mr. Trump. “I’ve always thought we were favored to, but I can’t say that now with all these cards up in the air.”

Republicans were taken aback this past week by the results of a 17-state survey commissioned by the Republican National Committee. It found the president struggling in the Electoral College battlegrounds and likely to lose without signs of an economic rebound this fall, according to a party strategist outside the R.N.C. who is familiar with the poll’s results.

The Trump campaign’s own surveys have also shown an erosion of support, according to four people familiar with the data, as the coronavirus remains the No. 1 issue worrying voters.

Polling this early is, of course, not determinative: In 2016 Hillary Clinton also enjoyed a wide advantage in many states well before November.

Yet Mr. Trump’s best hope to win a state he lost in 2016, Minnesota, also seems increasingly challenging. A Democratic survey taken by Senator Tina Smith showed the president trailing by 10 percentage points there, according to a Democratic strategist who viewed the poll.

The private data of the two parties is largely mirrored by public surveys. Just last week, three Pennsylvania polls and two Michigan surveys were released showing Mr. Trump losing outside the margin of error. And a pair of Florida polls were released that showed Mr. Biden enjoying a slim advantage in a state that is all but essential for Republicans to retain the presidency.

To some in the party, this feels all too similar to the last time they held the White House.

In 2006, anger at President George W. Bush and unease with the Iraq war propelled Democrats to reclaim Congress; two years later they captured the presidency thanks to the same anti-incumbent themes and an unexpected crisis that accelerated their advantage, the economic collapse of 2008. The two elections were effectively a single continuous rejection of Republican rule, as some in the G.O.P. fear 2018 and 2020 could become in a worst-case scenario.

“It already feels very similar to the 2008 cycle,” said Billy Piper, a Republican lobbyist and former chief of staff to Senator Mitch McConnell.

Significant questions remain that could tilt the outcome of this election: whether Americans experience a second wave of the virus in the fall, the condition of the economy and how well Mr. Biden performs after he emerges from his Wilmington, Del., basement, which many in his party are privately happy to keep him in so long as Mr. Trump is fumbling as he governs amid a crisis.

But if Republicans are comforted by the uncertainties that remain, they are alarmed by one element of this election that is already abundantly clear: The small-dollar fund-raising energy Democrats enjoyed in the midterms has not abated.

Most of the incumbent House Democrats facing competitive races enjoy a vast financial advantage over Republican challengers, who are struggling to garner attention as the virus overwhelms news coverage.

Still, few officials in either party believed the House was in play this year. There was also similar skepticism about the Senate. Then the virus struck and fund-raising reports covering the first three months of this year were released in mid-April.

Republican senators facing difficult races were not only all outraised by Democrats, they were also overwhelmed.

In Maine, for example, Senator Susan Collins brought in $2.4 million while her little-known rival, the House speaker Sara Gideon, raised more than $7 million. Even more concerning to Republicans is the lesser-known Thom Tillis of North Carolina. Republican officials are especially irritated at Mr. Tillis because he has little small-dollar support and raised only $2.1 million, which was more than doubled by his Democratic opponent.

“These Senate first-quarter fund-raising numbers are a serious wake-up call for the G.O.P.,” said Scott Reed, the top political strategist at the U.S. Chamber of Commerce.

The Republican Senate woes come as anger toward Mr. Trump is rising from some of the party’s most influential figures on Capitol Hill.

After working closely with Senate Republicans at the start of the year, some of the party’s top congressional strategists say the handful of political advisers Mr. Trump retains have communicated little with them since the health crisis began.

In a campaign steered by Mr. Trump, whose rallies drove fund-raising and data harvesting, the center of gravity has of late shifted to the White House. His campaign headquarters will remain closed for another few weeks, and West Wing officials say the president’s campaign manager, Brad Parscale, hasn’t been to the White House since last month, though he is in touch by phone.

Then there is the president’s conduct.

In just the last week, he has undercut the efforts of his campaign and his allies to attack Mr. Biden on China; suddenly proposed a halt on immigration; and said governors should not move too soon to reopen their economies — a week after calling on protesters to “liberate” their states. And that was all before his digression into the potential healing powers of disinfectants.

Republican lawmakers have gone from watching his lengthy daily briefings with a tight-lipped grimace to looking upon them with horror.

“Any of us can be onstage too much,” said the longtime Representative Greg Walden of Oregon, noting that “there’s a burnout factor no matter who you are, you’ve got to think about that.”

Privately, other party leaders are less restrained about the political damage they believe Mr. Trump is doing to himself and Republican candidates. One prominent G.O.P. senator said the nightly sessions were so painful he could not bear watching any longer.

“I would urge the president to focus on the positive, all that has been done and how we are preparing for a possible renewal of the pandemic in the fall,” said Representative Peter King, Republican of New York.

Asked about concerns over Mr. Trump’s briefings, the White House press secretary, Kayleigh McEnany, said, “Millions and millions of Americans tune in each day to hear directly from President Trump and appreciate his leadership, unprecedented coronavirus response, and confident outlook for America’s future.”

Mr. Trump’s thrashing about partly reflects his frustration with the virus and his inability to slow Mr. Biden’s rise in the polls. It’s also an illustration of his broader inability to shift the public conversation to another topic, something he has almost always been able to do when confronted with negative story lines ranging from impeachment proceedings to payouts to adult film stars.

Mr. Trump is also restless. Administration officials said they were looking to resume his travel in as soon as a week, although campaign rallies remain distant for now.

As they look for ways to regain the advantage, some Republicans believe the party must mount an immediate ad campaign blitzing Mr. Biden, identifying him to their advantage and framing the election as a clear choice.

“If Trump is the issue, he probably loses,” said Mr. Black, the consultant. “If he makes it about Biden and the economy is getting better, he has a chance.”