President Trump and his Republican allies are launching an aggressive strategy to fight what many of the administration’s own health officials view as one of the most effective ways to make voting safer amid the deadly spread of Covid-19: the expanded use of mail-in ballots.

The scene Tuesday of Wisconsinites in masks and gloves gathering in long lines to vote, after Republicans sued to defeat extended, mail-in ballot deadlines, did not deter the president and top officials in his party. Republican leaders said they were pushing ahead to fight state-level statutes that could expand absentee balloting in Michigan, Minnesota, Arizona and elsewhere. In New Mexico, Republicans are battling an effort to go to a mail-in-only primary, and they vowed on Wednesday to fight a new move to expand postal balloting in Minnesota.

The new political effort is clearly aimed at helping the president’s re-election prospects, as well as bolstering Republicans running further down the ballot. While his advisers tend to see the issue in more nuanced terms, Mr. Trump obviously views the issue in a stark, partisan way: He has complained that under Democratic plans for national expansion of early voting and voting by mail, “you’d never have a Republican elected in this country again.”

At his daily news briefing on Wednesday, Mr. Trump said he believed vote-by-mail had been abused to hurt Republicans, and “I will not stand for it,” though he allowed that mail ballots could help some older voters — an important part of his voting base. It was a slight modulation that came at the urging of his advisers.

He expanded on the idea on Twitter on Wednesday evening, calling absentee ballots “a great way to vote for the many senior citizens, military, and others who can’t get to the polls on Election Day.” He added that universal mail-in voting “shouldn’t be allowed!”

In their efforts to fight expanding vote-by-mail, Republican officials are counting on a crucial and powerful ally: like-minded judges. This week, conservative majorities on the U.S. Supreme Court and the highest court in Wisconsin indicated they did not view the pandemic as cause to yield on ideology, issuing party-line rulings rejecting Democratic efforts to defer Tuesday’s vote or extend mail-in balloting.

The decisions seemed to augur a hard road for Democrats in the looming court fights over how to proceed with voting in this crisis moment.

“We know moving forward that we will face a hostile court, that’s not new,” said Tom Perez, the chairman of the Democratic National Committee. But he indicated that it was just the beginning of a prolonged fight. “What they have done is invited further litigation, because they literally, quite literally, disenfranchised voters who had relied in good faith on the court.”

Ronna McDaniel, the Republican Party chairwoman, argued in an interview on Fox Business that the Democrats “want to take away the safeguards that ensure the integrity of the election process,” accusing them of trying to force “election reforms across the country during a time of crisis.”

The push to limit voting options is in keeping with Republicans’ decades-running campaign to impose restrictions that disproportionately affect people of color, the poor, and younger voters, under the banner of combating voter fraud — which is exceedingly rare. Democrats have more core constituencies among the nation’s disenfranchised, and both parties have long believed that easier voting measures will benefit Democrats.

But the current public health crisis brings new urgency to the battle, as Democrats and some Republican state officials turn to expanded voting by mail as an important way to avoid the serious health hazard of crowded polling stations amid a pandemic.

In a pre-coronavirus world, Republicans found that the specter of voter fraud and the need for tighter voter restrictions were popular messages with segments of their base. If there was a chance that the political equation might change with the pandemic, Mr. Trump and his allies have not seemed concerned.

The president has embraced some of the most outlandishly false claims about voter fraud, at times proclaiming that the popular vote in the 2016 election — which he lost — was “rigged.” He has long impugned voting by mail, which, while more vulnerable to fraud than in-person voting, has proved overwhelmingly secure in states with mail-in elections, including Colorado and Washington State. (Mr. Trump had formed a special commission to investigate voter fraud in 2016 but it produced no evidence before he shut it down in 2018.) Even so, he applied for his own mail-in ballot in Florida in March.

He has remained consistent in his opposition to mail-in voting throughout the coronavirus crisis, even as his lead health agency in the pandemic, the Centers for Disease Control and Prevention, lists postal balloting as its first recommendation to reduce crowd size at polling stations.

Mr. Trump has also argued against the second C.D.C. recommendation, encouraging early voting periods, saying they were “not the greatest because a lot of things happen.”

Advisers to Mr. Trump say that he has been set off by Democratic efforts, led by Speaker Nancy Pelosi, to include more aggressive efforts to promote absentee voting in a version of the coronavirus relief legislative package, which the Republican National Committee has equated with a “one-size-fits-all” approach that infringed upon states’ rights.

“States should maintain primary responsibility for their voting laws, making changes where necessary and utilizing flexibility within their existing laws,” said Justin Clark, the senior counsel to the Trump campaign.

Perhaps wary of the politics of taking an absolutist position amid the pandemic, and aware that absentee ballots can also be a preferred form of voting for some of Mr. Trump’s supporters, advisers to the president said that he would adapt his position in the coming days — like his nod to older voters on Wednesday — to acknowledge that absentee voting is acceptable, but that it must conform to the laws passed by specific states.

In practice in Wisconsin, Mr. Trump’s position led to a clear, partisan advantage for the state Republicans, who had appeared to brazenly trade public health for the political gain of lower turnout in Democratic bastions like Milwaukee.

Though results will not be released until April 13, simple turnout numbers in Milwaukee indicate that the Republican gambit worked. The city, which in 2016 had more than 167,000 people cast their ballots during the spring elections, most likely saw less than half that on Tuesday: Only 18,803 voted in person, and another 56,000 or so returned their absentee ballots. Though some may still be in transit, it is unlikely that Milwaukee, a Democratic stronghold in the state, will come close to matching its 2016 numbers.

Mr. Trump narrowly won Wisconsin in 2016, and both parties view the state as pivotal in 2020 — “the tipping point state,” Mr. Perez has called it.

The president followed Tuesday’s election closely, going so far as to weigh in several times to urge voters to support the State Supreme Court justice Daniel Kelly, an extraordinary, down-ballot focus for a sitting president.

Mr. Trump has taken such a keen interest in the race, advisers said, in part because Senator Ron Johnson of Wisconsin had spoken to him several times about the significance of such local campaigns — and because it represents a distraction from the grimness of the coronavirus that has gripped the country.

But Mr. Kelly is also among the five conservative justices on the seven-member State Supreme Court, and his re-election would preserve that comfortable balance for Republicans as legal wrangling over voting continues. The low turnout was viewed as a boon for his prospects to stay on the court.

“Nothing that Republicans do in Wisconsin is not part of the Trump campaign’s re-election strategy,” said Ben Wikler, the chairman of the Democratic Party of Wisconsin.

Mark Jefferson, the Wisconsin Republican Party executive director, said changing the election date would have been too chaotic. “We had passed the point of no return from a practical standpoint a few weeks ago,” he said.

For both parties, the battle began before the election Tuesday, and before the coronavirus froze the 2020 campaign. Mr. Perez, the D.N.C. chairman, established a voter protection program last year, one of his “four categories” of party infrastructure, and has put “voter protection directors” on the ground in 17 states.

The D.N.C. is also involved in lawsuits across the country, ranging from ballot placement issues in Georgia, Arizona and Texas to a delay in changing election procedure in Kansas. In Arizona, the D.N.C. won a case overturning a law that prevented voters from delivering the ballots of their neighbors, a process derisively known as “ballot harvesting.”

Ballot collection is also a key part of litigation that the Democratic super PAC Priorities USA has brought in Michigan, where it is fighting restrictions on submitting ballots on behalf of others. Aneesa McMillan, a spokeswoman for the group’s voting initiative, said that the provision could take on added significance as more Americans become homebound and infirm.

Republicans were highly focused on stopping Democrats from loosening such restrictions, which they have portrayed as a Democratic ruse to inflate voting tallies. As Mr. Trump put it Wednesday night, “thousands gathered and they come in and dump the location and you lose elections.”

In New Mexico, the national party was assisting its local arm in suing to block a statewide postal ballot. And, on Wednesday, the Minnesota Republican Party chair, Jennifer Carnahan, indicated she would fight a proposal by Secretary of State Steve Simon, a Democrat, to move to expand postal balloting, calling it a “power play by Simon to steal the free and fair election process of our state and country.”

For their part, public health officials were urging Republicans to drop their resistance to measures that could protect the electorate.

The David Geffen School of Medicine at U.C.L.A. has joined with the school’s Voting Rights Project to call for “universal vote by mail.”

“Everything is about minimizing risk and minimizing exposure,” said Jennifer Kates, director of global health and H.I.V. policy at the Kaiser Family Foundation. “Sending people to go vote in person is really the opposite of that.”

She added, “This is bigger than politics.”