Tillis, a North Carolina Republican who is retiring and has criticized Trump before, made the comment during an interview on CNN's State of the Union. Paxton defeated Cornyn in the May primary.
"John Cornyn's a great member of the US Senate. Paxton has his challenges," Tillis said. He argued that without Trump's involvement, Republicans wouldn't be facing a costly fight to hold the seat.
"We would not be spending tens or hundreds of millions of dollars in Texas if Cornyn had won," Tillis said.
The comments put a sitting Republican senator on record breaking with the president over a primary outcome that reshaped one of the biggest Senate races in the country. Cornyn had built a long career in Washington, and Tillis's remarks suggest some in the party see the general election as tougher, and pricier, than it needed to be.
Tillis, who isn't running for reelection, has little to lose by saying so out loud. Whether other Republicans agree with him in private is a different question entirely.