Senate Republicans, whose fragile majority has been threatened by Donald Trump’s tanking presidential bid, are suddenly confronting another problem: a lack of cash.

Republicans are set to be massively outspent on TV ads in seven of the eight states that are likely to decide control of the chamber. The spending disadvantage could badly hinder the GOP’s prospects, and it has led to growing frustration among the party’s top strategists — many of whom are convinced it’s long past time to cut Trump loose and focus almost exclusively on preserving the Senate majority.

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Republicans say they are particularly concerned that Democrats will use their financial advantage to tie the GOP candidates to an increasingly toxic Trump, who is now besieged by numerous accusations of sexual assault. In the New Hampshire Senate race — where Democrats have seized on GOP Sen. Kelly Ayotte’s errant debate comment that Trump “absolutely” was a role model for children, which she hastily retracted — Democrats have booked nearly $16 million of TV airtime between Oct. 11 and Nov. 8, while Republicans have set aside over $12 million, according to a media tracking source. In Indiana, where another key contest is unfolding, Democrats are set to air over $7 million worth of commercials during the same time frame, while Republicans have booked around $4 million.

And in Pennsylvania, the gap is particularly large. Democrats, who tied Republican Sen. Pat Toomey to Trump in another new ad last week, have reserved $17 million of TV airtime there — more than double the $8 million Republicans are set to air.

It could get worse. As Hillary Clinton pulls away from Trump, Republicans are worried Democrats will shift resources down-ballot. The Democratic super PAC Priorities USA, which has been dedicated to electing Clinton, is considering investing in several Senate races.

“I think it’s very disconcerting,” said Steven Law, who oversees Senate Leadership Fund, the principal outside group defending the Republican majority. “What we’re seeing is, Democrats’ big money has concluded that the presidential race is banked. So now, it’s pouring into the most competitive Senate races.”

Law, a longtime ally of Senate Majority Leader Mitch McConnell, said there was “almost giddy donor enthusiasm on the Democratic side.”

Despite their current advantage in TV reservations for the final weeks, Democrats still fear a last-minute infusion of cash into Republican coffers that could help tilt key races in the GOP's favor.

"As Trump gets increasingly more erratic and toxic, Democrats have to brace for the potential onslaught of Republican money being redirected to the Senate races to rescue their vulnerable incumbents," said Sadie Weiner, a spokeswoman for the Democratic Senatorial Campaign Committee. "It's important that our donors step up to help us do what we can to fight back."

Law’s group has already benefited from a handful of extremely large donations this year. Senate Leadership Fund and its affiliated nonprofit raised a whopping $42 million in August, including $20 million from Las Vegas casino magnate Sheldon Adelson and his wife, Miriam.

Yet, in the final stretch, Republicans risk being outspent – including in conservative states where their incumbents were once thought to be safe but are now under duress. In Missouri, Democrats have invested over $8 million, while Republicans have reserved just over $6 million. In North Carolina, Democrats have booked nearly $12 million, while Republicans have set aside around $9 million.

The GOP is scrambling. Last week, a number Republican groups, including the Club for Growth and Chamber of Commerce, held a conference call to discuss how to rush more money into Pennsylvania, where Toomey is fighting for his political survival. McConnell, meanwhile, has been urgently reaching out the party’s top donors and encouraging them to give to Senate Leadership Fund.

McConnell has also been leaning on his members. At a private lunch last month, he urged GOP senators to fork over more cash to the National Republican Senatorial Committee, which has been outraised by its Democratic counterpart by nearly $40 million. The lunch, which was held at the committee’s Capitol Hill headquarters, brought in $3 million, most of which came from South Dakota Sen. John Thune, the third-ranking Senate Republican.

Law, meanwhile, said he’d alerted his group’s donors “to this new surge of financial activity” from Democrats.

“My hope is that we’ll be able to continue to be competitive as we have been up to the last several weeks,” he added. “But whether we can close the gap entirely, I still don’t know.”

Republicans say it’s not too late to place additional reservations. According to one person familiar with Senate Leadership Fund’s plans, the group is likely to expand its TV bookings. The NRSC, meanwhile, is expected take out a loan, according to an aide, which would allow it to increase its late Senate spending. The organization also recently received a transfer of more than $4 million from the Republican National Committee.

But there are challenges. Two senior party strategists expressed frustration that Charles and David Koch, the billionaire industrialists who are among the party’s most generous benefactors, are not funding a fall TV campaign. Freedom Partners, a Koch-allied political group, has yet to reserve commercial time for the final four weeks leading up to the election. The Koch network has said that after spending tens of millions on commercials earlier in the campaign, it would be focused on ground game efforts during the final month, which they believe is a more cost-effective method of persuading voters.

The U.S. Chamber of Commerce, meanwhile, has only reserved about $650,000 for the final stretch — a small sum for such a powerful organization.

Instead, Republicans are largely relying on a trio of McConnell-allied entities: Senate Leadership Fund, Granite State Solutions (which is backing Ayotte in New Hampshire), and One Nation (a nonprofit group also run by Law). The three groups will account for over half of all the party’s TV spending during the final month.

“You never want to be in the red on a spending chart, but a lot of the groups that helped make a Senate majority back in 2014 are no longer on the field,” said Scott Jennings, a former top political adviser to George W. Bush. “You basically have Mitch McConnell and his groups against Barack Obama, Hillary Clinton and all that that entails.”

Others are questioning the approach of the NRSC, which spent heavily early on in this year’s campaign but now is short on cash for the final push. As of Friday afternoon, the committee had reserved just $5 million for the four week stretch — a small fraction of the $40 million booked by the DSCC. NRSC officials say they decided to spend money earlier in the campaign season in order to define Democratic candidates.

One national Republican strategist, however, called the decision a “giant gamble” and “a bit of a headscratcher.”

But without the early spending, NRSC aides argue, races in blue-tinted swing states like New Hampshire and Pennsylvania might already be over. “We made our reservations early so this strategy was communicated to outside groups and we believe that if this early spending wasn’t done we would be in a very, very bad place right now,” one NRSC official said.

The group also argued the spending gap shows the hollowness of Democratic arguments about the Kochs and heavy political spending.

“For the last two years all we heard from Harry Reid is how Republicans were outspending them. Breaking news folks, Democrats were lying and have trouble counting,” NRSC spokeswoman Andrea Bozek said.

Others are blaming the shortfall on a depressed Republican donor base. With many of the party’s deepest-pocketed contributors unhappy about Trump’s nomination, many are convinced, there has been less enthusiasm about giving to down-ballot causes.

Last week, Texas senators John Cornyn and Ted Cruz hosted what Republicans had hoped would be a big-splash fundraiser at the home of Houston energy executive Wil VanLoh, to benefit six incumbents up for reelection. The result, however, was anything but. The event raised only a fraction of what was expected, said one attendee.

“There's still a lot of money on the sidelines, especially in Texas, due to the current state of the party and its nominee,” said Jay Zeidman, a Texas health care executive whose family has given extensively to the party.

Trump, he argued, “is dividing more than bringing us together.”

Steven Shepard contributed to this report.