The Republican Party’s deep-pocketed political spenders are stepping up to prevent the once unthinkable: a U.S. Senate loss in deep-red Texas.

A pair of GOP super PACs are dropping significant sums into the Lone Star State as Sen. Ted Cruz (R-TX) faces a challenge from Rep. Beto O’Rourke (D-TX) that’s become far too competitive for comfort. As a reflection of how real—if still unlikely—they are taking the threat of a Cruz defeat, the sums spent have suddenly skyrocketed.

Independent spenders have dropped about $890,000 in support of Cruz or in opposition to O’Rourke this year, according to Federal Election Commission data. Nearly two-thirds of those expenditures, about $560,000, came in the eight days prior to Sept. 6, according to a review of campaign finance data.

The spike in independent expenditures is a tacit recognition of Cruz’s vulnerability. White House budget director Mick Mulvaney privately acknowledged that reality last week in comments to donors, first reported by The New York Times.

“There’s a very real possibility we will win a race for Senate in Florida and lose a race in Texas for Senate, O.K.?” Mulvaney said. “I don’t think it’s likely, but it’s a possibility.”

Leading the charge to save Cruz are super PACs associated with Texans Are and the Club for Growth, which spent $211,000 and $200,000 respectively, on television spots in late August hitting O’Rourke. The Club’s ad, which attacked O’Rourke over his proposed use of eminent domain to redevelop parts of El Paso, was part of a seven-figure push by the group to retain Republican control of the Senate seat in the face of Cruz’s apparent vulnerability.

While the Club is a well-known national organization, Texans Are is a newer group, formed last year by wealthy Cruz allies. It’s bankrolled by a host of six-figure donors, including Cinemark founder Lee Roy Mitchell, Omni Hotels owner Robert Rowling, and industrial supply tycoon Richard Uihlein.

The two groups’ spending sprees in Texas come as polls in the race tighten considerably. The most recent survey of the race, an Emerson poll conducted in late August, showed Cruz leading by just a single point, well within the margin of error.

The White House has also tacitly acknowledged Cruz’s peril. Trump, who ran a bitter presidential primary campaign against the Texas senator in 2016, will travel to Texas in October to campaign for the one-time rival he once dubbed “Lyin’ Ted.” And, according to Politico, other national Republican groups and big-named donors may soon jump into the fray as well. The list includes Americans for Prosperity, the Senate Leadership Fund, and the newly-formed Senate Reform Fund, and Ending Spending.

An upset in Texas less than a year after Democrats flipped a Senate seat in Alabama would deal a body blow to Republican hopes of retaining the upper chamber majority, which would be crucial to efforts by President Trump to continue confirming federal judges at a record clip.

Mulvaney alluded to the Alabama race in his comments to donors. “The president asks me all the time, ‘Why did Roy Moore lose?’” Mulvaney said, referring to the former state supreme court justice who was accused of sexual misconduct involving underage girls. “That’s easy. He was a terrible candidate.”

Though not accused of anything so scandalous, Cruz is notoriously disliked by his Senate colleagues, a point Mulvaney subtly acknowledged in raising the prospect of an O’Rourke victory.

“How likable is a candidate?” Mulvaney said. “That still counts.”