Republicans are growing increasingly worried that President Trump slide in the polls following his COVID-19 diagnosis, coupled with an outbreak at the White House, is posing a major threat to their Senate majority.

The presidential campaign has quickly become one of the most tumultuous in modern history, but theres more than enough turmoil and uncertainty to go around as both parties battle for control of the Senate.

One of the main concerns for Senate Republicans is Trumps cash crunch, which has forced him to cut back on advertising in key battleground states at a time when Senate Democratic challengers are projected to significantly outraise GOP incumbents heading into the final stretch.

Another challenge for Republicans is the expanding battleground map, with traditionally red states like Alaska, Kansas and South Carolina becoming more competitive as Democratic incumbents in Michigan and New Hampshire build comfortable leads.

Republicans are defending 23 seats, while Democrats only need to protect 12.

One GOP senator who requested anonymity to speak freely on the likelihood of Republicans losing the majority said Trumps poor numbers are a serious headwind.

Trumps not doing so well in some states he won handily four years ago, the lawmaker said, so we worry right now.

Senate Republicans were shocked by Trumps performance at the first debate, where he constantly interrupted moderator Chris WallaceChristopher (Chris) WallaceBob Dole claims no Republicans on debate commission support TrumpDebate commission co-chair: ‘No evidence whatsoever’ Trump has tested negativeTrump calls into Rush Limbaugh’s show for two hoursMORE and Democratic nominee Joe BidenJoe BidenDemocratic poll shows neck-and-neck race brewing in Florida House districtNebraska district could prove pivotal for Biden in NovemberBringing Black men back homeMORE and refused to condemn white supremacists.

Another bombshell came a few days later when Trump announced he had tested positive for COVID-19. As of Thursday, more than 30 White House staffers and people who had come into contact with them had also tested positive.

The debate performance combined with Trumps handling of his own COVID-19 diagnosis allowed Biden to widen his lead over the president.

Biden’s lead jumped from 6 percentage points the day of the first presidential debate to nearly 9 points on Friday, according to the RealClearPolitics average of national polling data. His lead also surged from just 1 point in Florida to nearly 4 points, from 5 points to 7 points in Michigan and from 6 points to 7 points in Pennsylvania.

A Quinnipiac University poll in Iowa conducted Oct. 1-5 showed Biden opening up a 5 point lead in the Hawkeye State.

Steve Jarding, a Democratic strategist, said the presidents COVID-19 diagnosis carries dire political implications.

Thats the one issue, obviously, he doesnt want to talk about. Hes taken big pains from Day One to say its not an issue, its a flu, its a hoax, itll go away, there will be a miracle. And all of a sudden it came home to roost, Jarding said.

Theres nothing worse that could have happened to this president whos been trying to convince America that hes done a great job but he couldnt even do a great job in his own house, he said.

Trumps COVID diagnosis also largely eclipsed his nomination of Judge Amy Coney Barrett to the Supreme Court, which Senate Republicans were rallying around in late September in hopes of a momentum boost in the home stretch of the 2020 campaign.

Sen. Ted CruzRafael (Ted) Edward CruzSunday shows preview: Trump, top Republicans recover from COVID-19; stimulus bill remains in limboDemocrats warn voters: Don’t get complacent The Memo: Trump searches for path to comebackMORE (R-Texas) on Friday said Trump could win reelection by a big margin, but warned that Republican candidates could also get wiped out in a bloodbath.

I am worried. Its volatile, its highly volatile, he said on CNBCs Squawk Box.

Cruz added that if voters are feeling optimistic, Republicans could see a fantastic election. But if people are angry and theyve given up hope then it could be a bloodbath of Watergate proportions.

In Montana, a state Trump carried by 20 points in 2016, the president is leading Biden by 9 points, a drop thats causing GOP jitters over the fate of Sen. Steve DainesSteven (Steve) David DainesRepublican fears grow over rising Democratic tideGOP anxiety grows over Trump political roller coasterGOP struggles to play defense on Trump’s ObamaCare lawsuitMORE (R-Mont.) in his tough race against Gov. Steve BullockSteve BullockOVERNIGHT ENERGY: Pendley says court decision ousting him from BLM has had ‘no impact’ | Court strikes down Obama-era rule targeting methane leaks from public lands drilling | Feds sued over no longer allowing polluters to pay for environmental projects Pendley says court decision ousting him from BLM has had ‘no impact’Supreme Court rejects GOP effort to block mail voting in MontanaMORE (D).

In South Carolina, which Trump carried by 14 points four years ago, the president has a slim 5 point lead over Biden. Thats a problem for Sen. Lindsey GrahamLindsey Olin GrahamGraham says SC people of color can go anywhere in the state but ‘need to be conservative, not liberal’Sunday shows preview: Trump, top Republicans recover from COVID-19; stimulus bill remains in limboErnst: ‘It would be smart’ for Senate Judiciary Committee to be tested for COVID-19MORE (R-S.C.), whos in the toughest reelection fight of his career.

The nonpartisan Cook Political Report this past week moved the South Carolina race from lean Republican to the more uncertain toss up.

Trumps sliding popularity in Texas is creating more concerns for Republicans there. After winning the state by 9 points in 2016, Trump now has just a 1.5 point lead, according to an average compiled by FiveThirtyEight.com.

Sen. John CornynJohn CornynOvernight Health Care: Trump to be released from hospital | Doctor won’t answer key questions | CDC says virus can spread through airborne transmissionThe Hill’s Campaign Report: Trump to leave Walter Reed l Post-debate polls show Biden building big lead l Coronavirus concerns ahead of VP debateCornyn: Trump ‘let his guard down’ on coronavirusMORE (R) is now facing a formidable challenge from Democratic candidate and Air Force veteran M.J. Hegar, who announced raising more than $13.5 million in the third quarter, nearly eight times what she raised in the second quarter. Cornyn said Hegar basically wiped out his cash-on-hand advantage.

Ford OConnell, a Republican strategist, said Trumps actions in the final weeks of the campaign season will be a key factor in determining who controls the Senate next year.

He said the battle for the Senate majority is directly tied to the presidential race. If Trump wins their state, its very, very likely that theyre going to win their campaign as well.

If Trump loses their state, theyre probably going to lose as well, OConnell added, noting that Sen. Susan CollinsSusan Margaret CollinsConservative group launches .3 million ad buy to boost Barrett SCOTUS nominationGOP vows quick confirmation of Trump’s Supreme Court pick amid coronavirus turmoilHickenlooper raises .6M for Colorado Senate bidMORE (R-Maine), who has crafted a reputation for independence over decades in Washington, is the one exception to the rule.

The one person who is on her own little island is Susan Collins, he said.

Collins, who has been trailing in the polls for months, got a dose of good news this past week when a Bangor Daily News/Digital Research survey showed her trailing her Democratic challenger, Sara Gideon, by only 1 point.

A Quinnipiac University poll in mid-September showed Collins trailing by as much as 12 points.

But the clear trend of Gideon in the lead remains unbroken, Senate Democrats point out.

Senate Republican strategists admit the environment has become even more challenging for their candidates.

A month ago it was an entirely different world, acknowledged a GOP official who said its incredibly challenging to predict what will happen on Election Day given how turbulent the cycle has been.

The official predicted Republicans would regain some momentum when the Supreme Court confirmation hearings begin this week and the attention returns to Barrett, who has near-unanimous support in the Senate GOP conference and is a favorite of conservative and evangelical activists.

Democrats also have headaches of their own.

The biggest is in North Carolina, where Democrat Cal Cunningham, who is married and has two teenage children, apologized this past week for having an affair this summer with a consultant in California.

Before those revelations, Democrats were growing increasingly confident of a victory over Sen. Thom TillisThomas (Thom) Roland TillisErnst: ‘It would be smart’ for Senate Judiciary Committee to be tested for COVID-19The Hill’s Campaign Report: Trump campaigns on Rush Limbaugh show l Democrats question Trump’s mental fitness l Coronavirus stimulus in doubt before electionCunningham dodges questions about text message scandalMORE (R), and saw it as the most likely fourth seat pick up they would need to flip the Senate. Cunningham reported raising an astounding $28.3 million in the third quarter, before news of the affair.

Now, some handicappers think Iowa, where Democrat Theresa Greenfield has a small but consistent polling lead over Sen. Joni ErnstJoni Kay ErnstErnst: ‘It would be smart’ for Senate Judiciary Committee to be tested for COVID-19Trump fuels and frustrates COVID-19 relief talksThe Hill’s Campaign Report: A debate over debates | Wisconsin mail ballots must be in by Nov. 3 | Who won Wednesday’s VP debate?MORE (R-Iowa), is a better pickup chance for Democrats than North Carolina.

The Cal Cunningham throws a little bit of a curveball here because a lot of us were looking at that as almost a must-win for the Democrats and that was the logical fourth seat after Maine, Arizona and Colorado, said Kyle Kondik, a political analyst at the University of Virginias Center for Politics.

Kondik said its a setback for Democrats and that Iowa is probably a better Democratic pickup opportunity in the wake of the Cunningham news.

Senate Republicans control 53 seats, and Sen. Doug Jones (D-Ala.), who is trailing badly in the polls, is expected to lose in November. That means Democrats need to capture at least four Republican-held seats and the White House to win back the majority. If Biden loses to Trump, Democrats would need to capture five GOP seats.