Senate Republicans made little effort to hide their disappointment Tuesday with President Donald Trump‘s decision to endorse Texas Attorney General Ken Paxton over incumbent Sen. John Cornyn (R-TX).GOP lawmakers were quick to relitigate Paxton’s political baggage and warned that the president may have just cost himself a winnable Senate seat by snubbing Cornyn. The two are locked in a bitter fight for the GOP nomination that will be decided in a May 26 runoff.“If Paxton wins the primary, we’ll be for him. But you don’t have to be a rocket scientist to figure out the pathway for Paxton is there, but it’s more uphill and it will cost more,” said Sen. Lindsey Graham (R-SC), a Trump ally who estimated it would cost Republicans three times more in the general election if Paxton prevails later this month.

In a lengthy statement, Trump emphasized Paxton’s loyalty and Cornyn’s past criticism of him to explain why he decided to take sides in the primary. In doing so, he ignored a lobbying blitz from Senate Republicans focused on Paxton’s ethics scandals and polling showing him to be the weaker general election candidate.

Cornyn had been in a dead heat with Paxton as early voting got underway on Monday, and Cornyn came out slightly ahead when the two ran against each other in the March primary. But the Trump endorsement is expected to tilt the scales in Paxton’s favor, setting off a wave of upset from Republicans who believe the seat is now in play for Democrats.