Rep. Kevin Cramer (R-N.D.) in Bismarck in May 2016. (Charles Rex Arbogast/AP)

Republican Rep. Kevin Cramer of North Dakota said Thursday that he intends to run for Senate against Democratic Sen. Heidi Heitkamp, setting up a tough race for the incumbent in a state President Trump won by a huge margin.

Cramer, who had announced just a month ago that he was passing on the race, told reporters at the Capitol that he had a change of heart in part because of witnessing important legislation, including the tax cut bill, squeak through the closely divided Senate.

“It’s more the significance of the United States Senate and the fragility of the majority that motivates me to give greater weight to that larger issue than just simply my family,” he said.

With signs that the $1.5 trillion tax cut bill is growing in popularity, Cramer predicted that Heitkamp’s vote against it could prove “fatal” for her, and he said he’d probably be making it a focus of his campaign.

“The vote that mattered, she failed on,” he said.

Cramer’s kickoff announcement is scheduled for Friday evening in Bismarck.

Cramer has been a strong supporter of Trump, and the president encouraged him to run. His entry into the race is good news for Republicans as they seek to defend and expand their narrow 51-to-49 majority in the Senate.

Heitkamp is a top target, along with a handful of other Senate Democrats up for reelection in states Trump won big, including Missouri, West Virginia and Indiana.

In a statement, the Democratic Senatorial Campaign Committee dismissed Cramer’s candidacy.

“Washington Republicans are in for a rude awakening now that they’ve tipped the scales for a congressman who has shown time and again he’ll do what’s best for his own interests,” said DSCC spokesman David Bergstein. “Voters will see right through Congressman Cramer and hold him accountable for a self-serving record that has hurt North Dakotans.”

Cramer said that Heitkamp’s recent vote against moving forward on a late-term abortion ban “lit up the phones, frankly,” especially after news coverage in North Dakota showed that Heitkamp had high-fived Senate Minority Leader Charles E. Schumer (D-N.Y.).

“Her sort of celebratory motions afterward on the floor with Leader Schumer just sent hundreds of North Dakotans to contact me and encourage me,” Cramer said. “So Kris and I just decided we have to reconsider in light of that call, and so that’s what prompted us to take another look at it.”

Kris Cramer is Kevin Cramer’s wife.

A PolitiFact investigation determined that rather than a high-five, the encounter between Heitkamp and Schumer on the floor during the vote was simply an awkward greeting.