The endorsement comes as new polling shows either Republican could be headed for a razor-thin November showdown.
SEE ALSO: Trump endorses Paxton in Texas Republican primary, boosting his challenge to incumbent Sen. Cornyn
In a social media post, Trump called Paxton "an America First Patriot" and someone who has "always been extremely loyal to me."
"His endorsement, in my opinion, is the most significant endorsement in my lifetime," Texas Attorney General Ken Paxton said on Tuesday.
In his post, the president said Senator John Cornyn is a good man, but that he "was not supportive of me when times were tough."
Still, Cornyn refused to fold.
"I believe that in terms of our success in November, that Ken Paxton would be an albatross around the neck of our candidates and would likely lose to James Talarico," Senator John Cornyn said during a campaign event in Lubbock on Tuesday.
In a social media post, the four-term senator said he worked closely with Trump during both administrations and voted with him "more than 99 percent of the time."
"I think Paxton got a tremendous boost today with the president's endorsement," Texas State University Political Science Professor Michael Adams said.
Adams led a newly released statewide TSU survey that showed just how tight the general election race in November could be.
Among likely voters who were surveyed, Cornyn narrowly led Democrat James Talarico 45% to 44%, while Paxton is tied with Talarico at 45%.
With early voting already underway ahead of next week's May 26 runoff, the clock is ticking in this closely watched Senate battle.
"I don't think anyone should be presumptuous at this point. Senator Cornyn is still on the ballot," Adams said. "I'm sure a lot of people, the takeaway today is saying that it may be over, but again, they're counting votes, and it's over on Tuesday."
Trump had previously asked the non-endorsed candidate to drop out of the race, but he did not make that kind of a request in his endorsement post today.