The COVID-19 pandemic, tanking markets and prompting fears of recession, has blindsided the 2020 campaign season, imperiling President Trump’s reelection. But Senate Republicans working to keep their majority in November are eyeing a different threat -- former Vice President Joe Biden’s sudden revival in the Democratic primary race.

Biden, a doubted and dismissed phoenix rocketing out of the ash pile, went from left-for-dead to presumptive nominee in an unprecedented 11-day sweep. Just before the South Carolina primary Republicans had already begun celebrating the hoped-for nomination of Sen. Bernie Sanders -- and according to all the polls, Democratic voters were poised to help them. With Sanders as the nominee, House Republicans could hope to take the majority back, and Senate Republicans could preserve their majority, and maybe even grow it.

Yet the furious resuscitation of Biden’s political fortunes has not only positioned his party more strongly against Trump in the general election, but suddenly scrambled the Senate map. Four Republicans up for reelection are now officially behind their challengers (or their most likely challengers) by four percentage points or more. And Biden’s numbers against an incumbent Trump show he is stronger than Hillary Clinton ever was in 2016 against the insurgent outsider most Americans expected would lose.

More important than his wins against Sanders have been the underlying numbers behind Biden’s success this past week. In a majority of the primaries, he is winning a broad and deep coalition that threatens Republicans’ ability to hold the Senate and the White House. With black voters, suburban voters, white voters without a college degree, white voters with a college degree, union and non-union, Republicans and independents, Biden’s breadth of support is remarkable.

In just days, Republicans went from feeling bullish about preserving their majority in the upper chamber to suddenly staring at potential losses across the board in stark relief. Only Sen. Lindsey Graham was willing to be blunt, saying while he thought Trump still had the edge, Biden would be “tough to beat.”

The most vulnerable GOP incumbent is Sen. Cory Gardner of Colorado, who has run consistently behind John Hickenlooper in polling since the former governor announced a run against him. Another contender in this year’s Democratic primary who had refused a Senate bid but changed his mind when Biden surged is Montana Gov. Steve Bullock. He won his red-state race in 2016 when Trump did as well, and will now challenge Sen. Steve Daines.

And for the most bipartisan member of the U.S. Senate, this cycle started out well but has turned into the fight of Susan Collins’ life. A Public Policy Poll in Maine a year ago showed her with the edge over her likely opponent, 51%-33%, but she is now behind the Democrat there, 47%-43%. Collins’ approval rating is down among Maine voters, who chose Clinton over Trump in 2016 by 57%-33%. (Though her approval with Trump voters rose after impeachment to 59%-26%.)

A new PPP survey shows Arizona Sen. Martha McSally behind Mark Kelly, 47%-42%, which hasn’t budged much from her standing in PPP’s January survey when Kelly was besting her 46%-42%. The poll shows poor approve/disapprove (37%-46%) numbers for McSally, who lost her Senate race in 2018 against Kyrsten Sinema but was then appointed to the late Sen. John McCain’s seat. Independents in that poll chose Kelly, 50%-29%. In the new OH Predictive Insights poll, Kelly is ahead by 49%-42% with 8% undecided and independents favoring him, 58%-29%.

Trump is unpopular in Arizona, a state he needs to hold this fall, and Maine, which he lost in 2016 by only three percentage points. Trump's approval in Maine is 42%-56% and in Arizona it's 45%-51%.

In North Carolina, Democrats secured their preferred nominee when last week Cal Cunningham beat out a more progressive candidate Republicans were spending money to help nominate. Cunningham already leads incumbent Thom Tillis by 48%-43% in an NBC-Marist poll.

Biden leads Trump in polling in all three of these swing states -- Arizona, North Carolina and Maine.

In addition to the four most embattled senators, other campaigns will tax the time and money of the GOP in races that favor Republicans but Democrats could win in a wave election.

In Iowa, Sen. Joni Ernst has seen her approval drop 10 points in the last year, when she was at 57%-47%. While 41% of voters, according to the Des Moines Register/Mediacom Iowa poll, said they would definitely vote to reelect her, 31% said they would definitely not. Though the Hawkeye State is seen as more reliably Republican than a battleground, headwinds there for Ernst will require the GOP to invest heavily in a state President Obama won twice.

In Georgia, Biden will attempt to turn out the coalition that has made him the presumptive nominee -- suburban moderates alienated from the GOP along with African Americans and non-college white moderates. In large numbers that group would imperil two Republican U.S. Senate seats there. This year, as Georgia has continued to grow more purple, not only is Trump ally Sen. David Perdue running for reelection, but Sen. Kelly Loeffler, appointed to the seat of former Sen. Johnny Isakson (who retired for health reasons), must run in a special election this November as well.

Texas will see an energized Democratic electorate as the party increased its turnout there in 2018 by more than 100% and Sen. Ted Cruz won by only 2.6 percentage points. The Texas GOP is trying to register 1 million new voters and is urging donors who have long sent their money around the country to please keep it in the Lonestar State. While Cruz said he believes Trump and John Cornyn will win Texas, he admitted “it will be hotly contested.”

Even in Kansas, Republicans will also be spending money they don't want to. Barbara Bollier, who was a Republican until a year ago, has a much better chance of beating Kris Kobach should he win the GOP nomination now that Secretary of State Mike Pompeo has declined to run. Kobach lost his gubernatorial run in 2018 to Democrat Laura Kelly, who will be able to help Bollier’s campaign. While it isn’t likely Kansas will turn blue, precious resources will have to be deployed there.

The other unique liability Republicans contemplate -- but never discuss -- is the potential that Trump cannot help them at all, even in states and districts where he wins. Tim Carney, a conservative writer for the Washington Examiner, wrote in November 2019 that Trump drags down Republicans “like an anchor” because he makes Trump voters (but not Republicans) out of working-class independents and Democrats but makes Democratic voters out of Republicans and independents. Added to the mix is high Democratic motivation and turnout, which has left Republicans losing elections in 2017, 2018 and 2019 in states across the country, including red ones like Kentucky and Kansas. If voters, who still see Trump as the opposite of the establishment, turn out for the president -- but not Tillis -- this fall in North Carolina, Tillis loses. This is because the voters someone like Tillis needs to count on are gone. “While Trump didn’t bring working class white-America into the GOP, he has caused a partisan realignment elsewhere: driving upper-middle class white America out of the GOP,” wrote Carney.

But Republicans are stuck with Trump. Stray and lose the base, or stay and fear the low ceiling as former Republicans stay home or vote Democratic. Currently, GOP senators are not only tied to him but most of their approval numbers are stuck in the low 40s, as are the president’s. And the answer to the coattails question will decide the Senate majority in November.

To mitigate against losses, Republicans are hoping Sen. Doug Jones will surrender his Senate seat in Alabama -- very likely -- and that John James can topple Sen. Gary Peters in Michigan. Biden’s performance in Tuesday’s primary is good news for Peters, not James. Voters in reliably Republican Livingston County, outside of Detroit, turned out in droves for Biden Tuesday, worrying Republicans who see Biden assembling the same coalition that elected Democrat Rep. Elissa Slotkin to that district in 2018 in a suburban-fueled wave. Trump had won the county by 30 points in 2016 but turnout increased there by more than 50% this week. And the day before Michigan voted, the Republican mayor of the all-important Macomb County town of Sterling Heights -- home of the “Reagan Democrats” whom Obama and then Trump won -- announced that though he voted for Trump in 2016 he was now supporting Biden.

Collectively there are headwinds facing all Senate GOP incumbents, from the coronavirus to the impeachment trial Senate Republicans held without witnesses. It was an audacious gambit for Majority Leader Mitch McConnell, who knew a sham trial -- in defiance of between 65% and 75% of the public wanting witnesses -- could cost Republicans their majority. Besides Collins’ vote in favor of witnesses, none of the vulnerable Republicans inoculated themselves with support of censure or even statements criticizing the president's conduct. Diagnosing Biden with dementia, or investigating his son Hunter, may not be enough to stop the bleeding.

This week a Quinnipiac poll comparing Biden to Trump showed why Republicans prayed for Sanders. On the question of who could better handle a crisis, Biden beat Trump, 56%-40%. On the question of whether they are honest, Biden beat Trump, 51%-33%. On the question of who cares for average Americans, Biden beat Trump, 59%-43%.

Biden’s appeal to a wide range of voters, who are turning out in surprisingly high numbers, shows voters are afraid of a second term of Trump. Republicans hoped Sanders the socialist would be scarier. Biden has problems as a candidate, and he may not win. But right now the only people Joe Biden scares are Republican incumbents.