After a heated and expensive campaign, the Republican Senate race in Texas between Senator John Cornyn and state Attorney General Ken Paxton will culminate in a runoff on Tuesday.

Mr. Cornyn, who has had a long career in Texas Republican politics and who has been an occasional critic of President Trump, is fighting for political survival against Mr. Paxton, who has been a magnet for scandal and recently won the president’s endorsement, giving him a big boost heading into the final stretch.

The race has been full of twists and turns. Here are five big moments from one of the marquee G.O.P. contests of the 2026 election cycle:

April 2025: Mr. Paxton announces his primary challenge against Mr. Cornyn

After weeks of teasing a potential bid to unseat Mr. Cornyn, Mr. Paxton officially announced his run in an interview on Fox News and published his campaign website, which prominently featured a photo of him with Mr. Trump.

Why it mattered: Mr. Paxton’s candidacy ensured that Mr. Cornyn would have a high-profile, MAGA-aligned challenger running to his right. Though Mr. Paxton had undergone an impeachment trial in 2023 over corruption allegations, he survived and positioned himself as the preferred candidate of the conservative base in Texas. Immediately, the contest became a high-profile test of the mood of the G.O.P. in Mr. Trump’s second term.

July 2025: Angela Paxton, a Texas state senator, announces that she is seeking a divorce and accuses Mr. Paxton of adultery

Just a few months after Mr. Paxton’s campaign announcement, Ms. Paxton announced that she was seeking a divorce from Mr. Paxton “on biblical grounds.” She attributed the impetus for her decision to “recent discoveries,” and the divorce petition she filed in court said the “respondent has committed adultery.” Mr. Paxton said the relationship had been strained by the pressure of public life and requested privacy.

Why it mattered: Mr. Cornyn’s camp and his allies seized on the allegations and began using them against Mr. Paxton. Mr. Cornyn suggested that the primary would become “a test of character” for his opponent.

October 2025: Representative Wesley Hunt enters the race

Mr. Hunt, one of the first Black Republicans to represent Texas in Congress, entered the race relatively late, offering himself as an alternative to what he called “the blood feud between Ken Paxton and John Cornyn.” Mr. Hunt specifically targeted Mr. Cornyn’s campaign, saying that his first priority if elected would be to repeal a bipartisan gun control measure that Mr. Cornyn had helped negotiate.

Why it mattered: Mr. Hunt’s candidacy created a three-way race that raised the likelihood of a runoff for the top two finishers, which is triggered in Texas if no candidate wins a majority in the primary. Mr. Cornyn’s campaign immediately attacked Mr. Hunt once he announced his campaign, while Mr. Paxton’s campaign welcomed the new Republican challenger.

March 2026: Mr. Cornyn finishes first in the primary, but falls short of a majority

Mr. Cornyn, powered by a substantial cash advantage, finished with about 42 percent of the vote, just ahead of Mr. Paxton, who won more than 40 percent of the vote. Mr. Hunt finished a distant third and was eliminated.

Why it mattered: In some ways, this was one of the highest points of Mr. Cornyn’s campaign. He secured a first-place finish ahead of Mr. Paxton. At the same time, his showing was not enough to spare him a runoff, and it was not enough to win an endorsement from Mr. Trump that some allies had hoped would soon follow.

May 2026: President Trump endorses Mr. Paxton

Though the president initially considered backing Mr. Cornyn after the primary in March, Mr. Trump ultimately decided to back Mr. Paxton, praising the state attorney general’s loyalty and unwavering support.

Why it mattered: Mr. Trump’s endorsement continues to be the most powerful stamp of approval in Republican contests, even as the president’s approval rating among all voters has sunk to a second term low. Tuesday’s vote will be an immediate test of the value of the endorsement. A win by Mr. Paxton would be a victory for Mr. Trump. But in the eyes of some other national Republicans, it would weaken the party’s chances in the general election.