WASHINGTON — As John Hickenlooper, the former Colorado governor and current Democratic candidate for Senate, began another campaign event via Facebook Live last week, he stated the obvious to his virtual audience.

“The nature of campaigns has changed,” Mr. Hickenlooper said as he beamed his message out to the political world from his family room in a joint appearance with Kathleen Sebelius, the former Obama administration health and human services secretary who was back home in Kansas, to talk about coping with the novel coronavirus. “These times really are different, and we are going to be doing things differently on this campaign.”

Mr. Hickenlooper, who is hoping to oust Senator Cory Gardner, a Republican, is not the only one adjusting to a radically changed campaign reality. The sudden onslaught of coronavirus has upended the nation’s congressional races as many were just getting started, altering the political landscape in unpredictable ways and forcing candidates in the battle for the Senate and House to adapt to unique circumstances.

Campaign officials and strategists are trying to carefully game out the new reality. The crisis could prove to be a boost for incumbents who have a built-in advantage in providing services to constituents at a time when voters are on edge and in need. But it is also shining a potentially unflattering spotlight on Washington’s response to the pandemic, which could hurt lawmakers who were already facing an uphill climb to re-election.

While awaiting new polling and other information, it is difficult to gauge who stands to gain.

“There are multiple logical scenarios, but it’s too early to know,” said Nathan L. Gonzales, editor of the nonpartisan newsletter Inside Elections. “The response is just getting started and there won’t be enough race-specific data to make a sweeping conclusion for at least a few weeks.”

What is certain is that the Rotary Club lunches, community gatherings, door-knocking and fund-raising receptions that are ordinarily the lifeblood of congressional races are gone for now. They are being replaced with tele-town halls focused on how to contend with the pandemic, virtual fund-raising get-togethers and appeals to contribute not to campaigns, but to nonprofit community groups as incumbents and challengers try to stay relevant in a grim news cycle dominated by a single topic over which they have no control.

In one example, Senator Thom Tillis, a first-term Republican facing a difficult re-election fight this fall in North Carolina, has been holding daily conference calls for constituents to dial in with questions about the pandemic. They are a chance for Mr. Tillis, who polls show to be deeply unpopular in his state, to present himself more as a social worker tending to voters’ needs than as a politician clinging to his seat in a close race.

In Arizona, Senator Martha McSally, another embattled Republican, announced she would devote 15 days to raising money for the Salvation Army, not her political organization. Theresa Greenfield, a Democratic challenger in Iowa hoping to replace Senator Joni Ernst, a Republican, has been urging Iowans to donate to food banks.

Applying his unique background to the situation, Mark Kelly, a Democrat and former astronaut trying to oust Ms. McSally, has been offering tips on how to cope with isolation during long days spent at home based on his time in space.

Gov. Steve Bullock of Montana, who decided to run for Senate only in March after saying for months that he was not interested in the job, is at the center of his state’s response to the pandemic. As the only sitting governor running for the Senate this year, Mr. Bullock, who hopes to defeat incumbent Senator Steve Daines, a Republican, has the potential advantage of being in the spotlight as Montanans confront the outbreak, sparing him the typical struggle challengers face in trying to grab attention from a well-known incumbent.

Audio leaked out last week of a conference call between the nation’s governors and President Trump in which Mr. Bullock challenged the president on lack of testing supplies. And Mr. Bullock is appearing regularly on television to speak out about the situation in his state. Republicans concede he could gain from his high-profile leadership role, but warn it could also hurt him if the state response is deemed wanting or bungled.

For now, the situation has given Mr. Bullock a chance to portray himself as above the partisan fray, as his advisers insist he is not thinking in terms of the political ins and outs.

“There will be a time for a campaign, and he looks forward to it,” said Matt McKenna, the governor’s political adviser. “But right now he is focused on fighting this pandemic, keeping Montanans safe and getting front line workers the resources they need.

House contenders have also sought to emphasize their constituent work. Representative Harley Rouda, Democrat of California, rerouted campaign volunteers away from their usual calls, directing them to contact older adults for wellness checks instead. Representative Colin Allred, Democrat of Texas, has used his email list to send out fund-raising links to local food banks.

The National Republican Congressional Committee, normally a bastion for Trumpian name-calling and hard-edge partisan attacks, used its Twitter account on Friday to circulate a link to guidance for small businesses on how to obtain newly available loans through the just-enacted economic stimulus law. In a memo, the committee, the campaign arm of House Republicans, also urged candidates to watch their tone in messages to voters.

“At times like this, you need to ask yourself if your press release or snarky comment are in poor taste,” the memo said.

Some incumbents have already experienced the risks of being tied closely to the government’s response to the pandemic. During one tele-town hall, Mr. Tillis, who is facing a challenge from Cal Cunningham, a former Democratic state legislator, came under criticism from a constituent who said the economic relief measures enacted in Washington in recent weeks — including $1,200 direct payments to taxpayers — were not enough. While most of those who spoke sounded unconcerned with politics and more interested in learning how to collect unemployment benefits and other aid, a woman named Sarah lashed out at Mr. Tillis for what she argued were overly restrictive stay-at-home policies that she said were harming the economy and costing working families jobs. The nation’s elected leaders, she said, ought to be making hard decisions to minimize the impact of the coronavirus while keeping the country at work.

“Your one-time check to my family isn’t going to help us recover from what we are suffering right now,” said the woman, who declined to share her last name. “I just find this extended lockdown to be outrageous.”

Mr. Tillis offered a meandering answer, but stood firm in defense of the current social distancing program.

“If we send everyone back to work, I guarantee you the peak will be greater, the number of hospital beds will be fewer and people will die,” he said. “What we are trying to do is minimize that, flatten the curve and get back to work.”

The crisis has provided several Republican senators in highly competitive races the opportunity to emphasize their role in both fashioning the $2 trillion stimulus package and in helping secure needed medical supplies for their states. Mr. Gardner, who is trying to hold off Mr. Hickenlooper, said he used connections he made through his work on the Senate Foreign Relations Committee to help secure 100,000 masks for Colorado out of one million that Taiwan donated to the states.

Republican campaign officials said they were urging senators to focus on the crisis, rather than shift into campaign mode.

“The only guidance we have is be a senator,” said Kevin McLaughlin, executive director of the National Republican Senatorial Committee. “Stay in touch with people, be a point of reference, just go do your job.”

The Senate website of Mr. Daines, who had hoped to escape a contest with Mr. Bullock, reflects that approach. It features a compilation of favorable video clips from Montana news outlets about Mr. Daines’s role in the coronavirus aid legislation — including a White House shout out from Vice President Mike Pence — under the headline “Sen. Daines is fighting for Montana on Covid-19 ” Also prominent were releases about benefits he had pushed, including a timeline of his role.

Some analysts said they were skeptical that Senate incumbents would receive a bounce from the legislative package. Mr. Gonzales suggested that even in this extraordinary environment, the battle for control of the Senate would come down to the public perception of Mr. Trump.

“In the end, I expect voters to fall back to their partisan corners and the most competitive Senate races will be significantly impacted by the president’s standing,” he said.

Democrats are counting on that as well, pointing to public unhappiness with the president’s response.

“In this evolving crisis, people want reliable information and steady leadership,” said Lauren Passalacqua, a spokeswoman for the Democratic Senatorial Campaign Committee. “Democrats are focused on solutions to address the spread of coronavirus and mitigate its impact on their states, sharing resources, hosting virtual town halls with experts and working to support their communities.” One thing both sides agree on is that even given the stakes in the fight for congressional control, campaigning is hardly uppermost in anyone’s mind at the moment.

“We have to deal with this,” Mr. McLaughlin, the head of the Senate Republican campaign group, said of the pandemic. “If we don’t fix this, it just doesn’t matter.”

Nicholas Fandos and Catie Edmondson contributed reporting.