North Dakota Republican Rep. Kevin Cramer says he will be running for US Senate — a reversal from last month, when he said he would run for reelection in the House, and a major warning sign for Democratic Sen. Heidi Heitkamp, who is defending one of the most vulnerable seats in the 2018 midterm elections.

Cramer’s team put out a Facebook invitation for a “Kevin Cramer for U.S. Senate” rally Thursday for a Friday announcement ceremony in Bismarck, North Dakota. He is looking to unseat Heitkamp, one of 10 Democrats running in states President Donald Trump won in 2016.

Cramer, who is a close ally of Trump, was a top recruit for Republicans in North Dakota. Senate Majority Leader Mitch McConnell has reportedly been heavily recruiting the North Dakota lawmaker to run for the seat, identifying him as the best chance for Republicans to increase their slim majority in the Senate.

Cramer played a key role in crafting Trump’s energy platform during the election cycle and has had Trump’s ear on the issue since. A former utility regulator and climate change skeptic, he encouraged Trump to walk back the United States’ commitment in the international Paris climate agreement.

Until now, Republicans hadn’t been able to find a formidable opponent to run against Heitkamp. Now, Republican hopeful Gary Emineth, a former state party chair, withdrew from the race this week, citing Cramer’s intentions on running. State Sen. Tom Campbell is also running, but many worry he lacks the recognition to win state-wide office, and he could drop out as well.

Cramer is not without controversy. CNN reported on how he downplayed former White House press secretary Sean Spicer’s “Holocaust centers” remarks and once criticized “bad-looking white pantsuits” Democratic women wore to honor women’s suffrage. But that may play well in a state that went for Trump over Hillary Clinton by almost 36 points in the presidential election.

Democrats are facing a difficult Senate election map in 2018 and have been banking on Trump’s unpopularity to increase voter turnout on their side of the aisle. The energy among Democratic voters has made Republican organizers and strategists uneasy in recent months, after landslide Democratic special election victories in deeply red districts showed early signs of a blue wave in 2018. Reports show that some Republicans are even trying to convince Sen. Bob Corker (R-TN), who announced his retirement last year, to reconsider a reelection bid — even though he would face a very tough Republican primary.

Cramer’s decision to not run in the Senate raise gave Heitkamp some breathing room. But that’s all changed now, making North Dakota even more of a state to watch in the 2018 elections.