WASHINGTON — Republican control of the U.S. Senate could become another casualty of the coronavirus crisis.

The pandemic has upended 2020 politics — not just for President Trump — and his party increasingly worries that the turmoil has given Democrats fundraising and strategic advantages that put Republicans’ 53-47 Senate majority at risk.

Even as Republicans struggle to avoid being dragged down by Trump’s unsteady handling of the COVID-19 response, they’re facing an enthusiasm gap, at least among donors, that favors Democrats.

Fundraising reports show that Senate Democratic challengers far surpassed their Republican opponents in the first three months of 2020 — in large part because of superior online fundraising, much of it from small-dollar donors.

Online fundraising is a crucial tool for raking in donations while social distancing rules are in place, and Republicans, who were once confident of holding their Senate majority, concede that they’ve fallen behind.

“The Senate is absolutely at risk of going Democratic,” said Josh Holmes, a top GOP strategist and confidant of Senate Majority Leader Mitch McConnell (R-Ky.). “Democrats are still way ahead when it comes to digital fundraising. It will get worse. It will get a lot worse, because there is no easy way to play catch-up.”

The impact of the health emergency is visible across the landscape for incumbent Republicans. In Maine, endangered Sen. Susan Collins has turned some of her reelection ads into public service announcements about health risks and praise for essential workers. In Montana, Sen. Steve Daines is overshadowed in local media by his Democratic opponent, Gov. Steve Bullock, who leads the state’s coronavirus relief effort. In North Carolina, a Democratic ad hit Sen. Thom Tillis over comments questioning the need for hand-washing regulations for restaurant workers — an issue that previously would have drawn little notice.

To take control of the Senate, Democrats would need a net gain of four seats if Republicans keep the White House, but three seats if they oust Trump and the Democratic vice president becomes the Senate tiebreaker.

The balance of power hinges on four states with the most vulnerable Republicans up for reelection: Maine and North Carolina, as well as Colorado and Arizona. Because Democrats are likely to lose one of their own seats — Democrat Doug Jones is a long shot in deep-red Alabama — they would have to win all four of those Republican-held seats or offset losses with wins elsewhere. That’s a tall order, but less so than a year ago. In mid-April 2019, the nonpartisan Cook Political Report rated just two of those races — in Arizona and Colorado — as toss-ups. Now all four are.

But Democrats have put other states into play. In Montana, a recent poll for the first time gave Bullock a lead after the pandemic response efforts heightened his profile. A political scientist calculated that Bullock had 800 mentions in local media in March, compared with 150 for Daines.

Besides the coronavirus, Republicans are at the mercy of another force beyond their control: a mercurial and erratic president.

“Two months ago, there was a lot of confidence” about the Senate, said Alex Conant, a GOP strategist. “Now, no one knows what the world is going to look like in six months. How Trump handles the crisis will determine the outcome.”

A strategy memo circulated by the National Republican Senatorial Committee recommended that Republican senators not defend Trump’s handling of the COVID-19 response but instead attack China, accusing it of spreading the virus.

Another line of defense: Republican incumbents are urged to spotlight their Senate work to help address their states’ health and economic problems. Sen. Todd Young of Indiana, chairman of the Senate campaign committee, said there could even be a “real upside politically” from that approach.

“This could be quite beneficial toward our incumbents in their efforts to send a message that they are here for their constituents when they need them most,” Young told reporters at the Capitol this week.

Collins, by portraying herself as a problem-solving “mayor of Maine,” as one party strategist put it, could help rebuild her reputation as a political independent. Her approval ratings have plummeted in recent years in the wake of her support for Trump’s nomination of Brett M. Kavanaugh to the Supreme Court and her opposition to the impeached president’s removal from office.

Collins in March paused traditional campaign activities and switched up her television ads. Instead of touting her work on roads and diabetes research, she aired ads that urged voters to wash their hands and highlighted her role in passing small-business relief.

Her leading Democratic opponent, state House Speaker Sara Gideon, also changed her ads’ focus to public health. Gideon is helped by outside Democratic groups, who have continued to slam Collins for her support for Trump and the failings of federal relief programs.

In North Carolina, Tillis’ first digital ad of the general election season does not even mention his campaign. It focuses on his support for small-business relief and his appointment to a Trump task force to advise on reopening the economy.

Democrats have renewed attacks on Tillis’ record on health policies, including his opposition to expanding Medicaid as a state legislator. They circulated a 2015 video of Tillis saying he would not mind allowing restaurants to “opt out” of regulations requiring employees to wash their hands. He later told a reporter it was a joke to spark discussion about over-regulation.

Though Trump is something of a liability in competitive states, Republicans still need his support to rally party loyalists. Colorado Sen. Cory Gardner got a plug in early April when the president tweeted that the federal government would be sending 100 ventilators “at the request of Senator Gardner.” Critics said Trump’s move smacked of inappropriate political patronage.

Sen. Martha McSally in Arizona this week flew with Trump on Air Force One and accompanied him on a visit to an Arizona mask-making factory. Trump, who has tried to deflect blame for his management of the health emergency by blaming China for its origins, praised McSally for “fighting to uncover the full truth about the China situation.”

Republicans received a wake-up call mid-April when candidates’ first-quarter fundraising reports were released. The incumbents in all four of the most competitive states were out-raised by their Democratic challengers, by ratios of up to 3 to 1. Three of those incumbents kept an edge in cash on hand, but their advantage was sharply reduced.

The Republican senators suffered in part because they had to cancel many fundraising events in January during the Trump impeachment trial. Then in March, in-person fundraising was again put on hold with the imposition of social distancing and stay-at-home orders.

Democratic challengers showed their strength in online fundraising, especially of small contributions. In Maine, more than 40% of Gideon’s contributions came in donations of less than $200, compared with less than 16% of Collins’ money. Overall, Gideon raised three times as much money as Collins in the first quarter — $7.1 million to Collins’ $2.4 million.

In North Carolina, Democratic challenger Cal Cunningham raised more than twice as much as Tillis.

“We know full well that we will need to ramp up our fundraising,” Tillis campaign manager Luke Blanchat said.

In Arizona, McSally’s Democratic challenger — Mark Kelly, the former astronaut and husband of gun control advocate and former Rep. Gabrielle Giffords — has out-raised McSally in five consecutive quarters. As of the end of March, he had almost twice as much cash on hand.

Republicans will benefit from strong fundraising by their party and groups like the U.S. Chamber of Commerce and the Senate Leadership Fund, a super PAC close to McConnell that has already invested $67.1 million in TV ad time for this fall in six states.

“We think this is the week politics restarts,” said Scott Reed, a political advisor to the chamber. “The Senate is going to be a fight to the finish.”

Times staff writer Jennifer Haberkorn contributed to this report.