(CNN) Democratic challengers in some of the most closely watched US Senate races outraised Republican incumbents during the first three months of the year -- as the coronavirus outbreak upended traditional campaigning and fundraising.

Strategists in both parties say the fundraising environment will only grow more difficult as candidates face more weeks of the country being shut down by the global pandemic.

"There is no playbook for politics in a pandemic," said Josh Holmes, a Republican strategist and former top aide to Senate Majority Leader Mitch McConnell of Kentucky.

Democrats outpaced all four Republican incumbents in Senate contests rated as pure "toss-ups" by the non-partisan Cook Political Report . Campaigns were required to file their fundraising reports for the first quarter, which ended on March 31, with the Federal Election Commission by midnight on Wednesday.

Among the fundraising leaders: retired astronaut Mark Kelly , a Democrat seeking to oust Republican Sen. Martha McSally of Arizona, who was appointed to the Senate in late 2018. Kelly, a prominent gun-safety advocate and the husband of former Rep. Gabrielle Giffords, raised $11 million, expanding his fundraising lead over McSally, who brought in nearly $6.4 million between January 1 and March 31. Kelly had $19.7 million remaining in his campaign account at the end of the quarter, while McSally had nearly $10.3 million.

The top-tier contest in Arizona will help determine which party controls the US Senate after November's general election. Democrats must win a net gain of four seats to flip the chamber, or three if they win the White House and the vice president is a tie breaker in the Senate.

In other key races, Democrats posted big numbers.

In Maine, state House Speaker Sara Gideon, a Democrat running to take on GOP Sen. Susan Collins, raised $7.1 million during the first three months of the year.

That was nearly three times the $2.4 million in total receipts that went to Collins, a four-term senator who has emerged as a top target for Democrats angered by her votes for the Republican tax overhaul, to confirm Supreme Court Justice Brett Kavanaugh and to acquit President Donald Trump after his Senate impeachment trial this year.

Collins reported having $1 million more than Gideon left in the bank at the end of the quarter.

Former Colorado Gov. John Hickenlooper, who briefly sought the 2020 Democratic presidential nomination, outraised Republican Sen. Cory Gardner, bringing in roughly $4 million to Gardner's nearly $2.5 million

But Gardner started the second quarter with much more money in the bank: nearly $9.6 million to Hickenlooper's $4.9 million.

In North Carolina, Army veteran Cal Cunningham, who won the Democratic nomination on March 3, raised nearly $4.4 million between January and March -- far outpacing Republican Sen. Thom Tillis.

But while Cunningham raised more than double what Tillis did, the Republican incumbent still had a cash-on-hand advantage of $6.5 million to Cunningham's $3 million.

In Georgia, Democrat Raphael Warnock raised $1.5 million in the first quarter of 2020, outperforming Sen. Kelly Loeffler, the Republican appointed to the seat last year , and GOP Rep. Doug Collins, who also is running for the seat in November. Collins raised more than $842,000 and transferred $1.6 million from his House campaign account.

Loeffler, who has pledged to tap her personal fortune for the race, raised about $1.1 million and loaned her campaign $5 million on March 31, FEC filings show. She had a substantial sum -- $6.1 million -- remaining in her campaign war chest to spend in the months ahead.

In Montana, Democratic Gov. Steve Bullock raised more than $3.3 million in the three weeks between entering the US Senate race in early March and the end of the quarter. Republican Sen. Steve Daines raised $1.3 million but had built a far bigger war chest: $5.6 million to Bullock's nearly $3.2 million.

In Kansas' open-seat race, Democratic state Sen. Barbara Bollier raised nearly $2.4 million and had $2.4 million on hand, much more than her potential Republican challengers. Former Kansas secretary of state Kris Kobach raised over $242,000. Meanwhile, GOP Rep. Roger Marshall raised about $377,000 but claimed about $1.95 million in his campaign coffer at the end of the quarter.

In several other competitive contests, Republicans held their own against Democrats. In Iowa, for instance, Sen. Joni Ernst outraised Democratic challenger Theresa Greenfield , nearly $2.75 million to more than $2.25 million.

And in Michigan, Republican challenger John James, an African-American businessman and combat veteran backed by President Donald Trump, raised $4.8 million, besting Democratic Sen. Gary Peters for the third quarter in a row.

In the other Georgia Senate race, which the Cook Political Report rates as "likely" to remain in Republican hands, GOP Sen. David Perdue raised $1.65 million, while Democrat Jon Ossoff brought in a little more than $1 million.

The Cook Political Report also rates the Kentucky Senate race between McConnell and Amy McGrath, a retired Marine lieutenant colonel, as "likely Republican." But McGrath set a blistering fundraising pace, raising more than $12.8 million in the first quarter to McConnell's nearly $7.5 million haul.

Perfect storm: Impeachment and then a pandemic

Holmes said GOP incumbents in the Senate were hit with a double whammy as they sought to fundraise during the first three months of this year: An impeachment trial that kept senators in Washington and away from high-dollar fundraising events, followed by a global pandemic.

"It was kind of a perfect storm. You had a January that was erased (by the impeachment trial) and a March that was erased because of the coronavirus," he said. "Unless you had a campaign that was diversified in terms of how you were bringing in donations, it was a really tough quarter for incumbents."

Holmes said Democrats still have the edge in small-dollar, online donations, but he said the creation of WinRed, an online fundraising platform launched last year has helped Republicans "make giant leaps in this area." WinRed officials recently announced they had raised nearly $130 million online for Republicans in the first three months of this year.

Political fundraisers now fear that the pandemic will doom campaign donations in the second quarter, which runs from April to the end of June, as the economy crashes and donors' preference for in-person access gives way to never-ending conference calls.

Without Broadway or baseball, political fundraisers have had to come up with new ways to introduce their clients to donors and sway them to take out their checkbooks.

Michael Fraioli, a Democratic fundraising consultant, said that his clients stopped in-person meetings and events on March 11, the same day that the World Health Organization declared the outbreak a pandemic and a couple weeks before the first quarter deadline.

In the weeks since then, Fraioli said fundraisers have turned to video chats. Some campaigns have gotten creative, connecting clients from rival firms to team up and make joint pitches to prospective donors, he added.

"Zoom has probably never been so busy, at least in the world of politics," said Fraioli, referring to the suddenly ubiquitous software platform. (Of course, video conferencing has its drawbacks for candidates seeking donations: fewer people and less intimacy.)

Candidates have also had to find a softer touch. One Democratic fundraiser with congressional clients told CNN that they have urged their clients to check in on donors but not ask for money. "This is a time for compassion first and foremost," said the fundraiser. "The money can come later."

Fraioli said that challengers to incumbents in low-profile races could see a "significant drop off" in fundraising during the new era of social distancing.

"They probably don't have yet a significant digital operation," he said. "And calling individuals is hard on a good day."