US senator Thom Tillis on Sunday criticized Donald Trump, his fellow Republican, for the president’s support of Ken Paxton over incumbent John Cornyn in the Texas Senate Republican primary.

“He made a colossal mistake,” Tillis, a frequent Trump critic from North Carolina, said during an interview on CNN’s State of the Union in reference to the president’s support of Paxton who defeated Cornyn in the May primary.

“John Cornyn’s a great member of the US Senate. Paxton has his challenges,” the retiring Tillis added. “We would not be spending tens or hundreds of millions of dollars in Texas if Cornyn had won.”

Tillis pointed to the president’s involvement in the contest and described Trump’s decision to back Paxton instead of Cornyn as an “unforced error” that has put the Senate seat at risk for the GOP.

Trump endorsed Paxton just days before the Texas attorney general defeated Cornyn, a four-term senator, and advanced to face Democratic candidate James Talarico, a in the general election on 3 November.

The race had wide implications for Trump’s strength heading into November’s midterm federal elections. Paxton will now face Talarico, a Democratic pastor and state legislator whose message of peace and populism has attracted much attention. If he wins, Talarico would become the first Democrat in more than 30 years to win statewide office in Texas.

When he chose to support Paxton over Cornyn, Trump highlighted the Texas attorney general’s record as a steadfast supporter of the president’s Make America Great Again (Maga) movement.

Cornyn had previously said during the 2024 presidential campaign that Republicans should move on from Trump.

In a social media post shortly before the election day, Trump said Cornyn had been “VERY disloyal” to him and implored voters in Texas to “REMEMBER!”

Paxton, meanwhile, is struggling to overcome a series of political scandals. He was impeached in 2023 after being accused of corruption and later acquitted in a trial in the Texas senate. He was also indicted on charges of felony securities fraud, but the case was dismissed after a 2024 pretrial diversion agreement.

In July 2025, his wife of 38 years, Angela Paxton, filed for divorce “on biblical grounds”, citing adultery.

Tillis is one of a handful of Republicans to repeatedly break with Trump and publicly criticize the president’s decisions. In April, he spoke out in defense of the Nato military alliance after Trump said he was “absolutely” considering withdrawing the US from it.

“Any president that contemplates attempting to withdraw from Nato is not only fulfilling [Russian leader] Vladimir Putin and [Chinese premier] Xi Jinping’s greatest dreams but would be undermining America’s own national security interests,” Tillis said at the time.

He also called for then secretary of homeland security Kristi Noem’s resignation after immigration agents killed two US citizens in Minneapolis in January, calling her leadership “a disaster” during the last Senate hearing before her firing.

Tillis was one of a small number of Republicans on Thursday to vote in favor of a Democrat-led war powers resolution that aimed to end the US war with Iran. Despite gaining some Republican support, the proposal failed to pass in the Senate by a 49-50 vote.