Donald Trump endorses U.S. Sen. John Cornyn for re-election

President Donald Trump endorsed the re-election bid Thursday of U.S. Sen. John Cornyn, R-Texas, who is seeking his fourth term in 2020.

"Senator John Cornyn has done an outstanding job for the people of Texas," Trump tweeted. "He is strong on Crime, the Border, the Second Amendment and loves our Military and Vets. John has my complete and total endorsement. MAKE AMERICA GREAT AGAIN!"

Cornyn, speaking with reporters in Austin after the president tweeted, said he asked Trump for an endorsement.

"I did ask him for it, and I appreciate his support," he said. "Obviously, we know that 2020 is gonna be another challenging election."

The endorsement, 21 months before Election Day, marks the latest move that shows Republicans are taking Cornyn's re-election bid seriously — something the senator quickly acknowledged to reporters Thursday afternoon. After a closer-than-expected U.S. Senate race in November between Ted Cruz and Democratic challenger Beto O'Rourke, the state's minority party has eyed Cornyn's seat as a potential pickup in 2020. Last month, Steve Munisteri, a former Texas GOP chairman and longtime Republican fixture in the state, announced he was leaving the White House to join Cornyn's campaign as a senior adviser. Cornyn said Thursday that his team is "staffing up" and "trying to learn the lessons of 2018 [and] listening to what the voters are telling us."

Still, it's unclear who on the Democratic side may end up challenging Cornyn. O'Rourke, who has said he plans to make a decision about a potential presidential bid by the end of the month, has also been in talks with senior Democrats about challenging Cornyn, according to Politico. Cornyn quickly responded to such reports by sending a fundraising email earlier this week announcing a "STOP BETO FUND," telling supporters "we need to be ready for anything."

Beyond O'Rourke, Democrat MJ Hegar, a military veteran who unsuccessfully challenged U.S. Rep. John Carter, R-Round Rock, last year, told The Texas Tribune that she's considering a run.