Controversial former coal baron Don Blankenship said Monday that he is launching a third-party campaign for the Senate after losing in West Virginia’s Republican primary, a decision that could complicate the GOP’s efforts to keep its majority in the midterm elections.

Blankenship, who finished third in the Republican primary for Senate in West Virginia earlier this month, said in a statement that he has accepted the Constitution Party nomination. But he must convince state election officials that his campaign does not run afoul of a “sore loser” law barring candidates who lose in party primaries from later switching their party affiliation to get on the general election ballot, which could be a difficult challenge.

If Blankenship is able to do that, it would be a blow to the Republican nominee, state Attorney General Patrick Morrisey. Blankenship has the ability to attract some support from conservative voters that Morrisey is trying to consolidate behind his campaign.

“It is especially appropriate for me to be nominated by the Constitution Party given its staunch and uncompromising commitment to upholding the United States Constitution,” Blankenship said in a statement.

Blankenship’s candidacy in the GOP primary alarmed Republican officials. He served a year in prison for conspiring to violate mine safety and health standards after a 2010 underground explosion killed 29 miners. During the campaign, he used racial epithets to describe black people and those of Chinese descent.

Top Republicans worried that if he won the party’s nomination, it would prevent them from competing for the Senate seat in the general election. President Trump and Senate Majority Leader Mitch McConnell (R-Ky.) worked together to help prevent Blankenship from winning the nomination. The day before the May 8 primary, Trump urged West Virginia Republican voters not to cast their ballots for him.

“We were assured by White House political staff that they would not interfere in the primary election. Obviously, that turned out not to be true,” Blankenship said. A White House official did not immediately respond to a request for comment.

Morrisey won the GOP nomination to face Democratic Sen. Joe Manchin III. Manchin is regarded as one of the most vulnerable senators facing reelection this year. Trump defeated Hillary Clinton in West Virginia by about 42 points in the 2016 presidential election.

Republicans are defending a slim 51-49 majority in the November midterms. McConnell said in an interview with The Washington Post last week that he believes West Virginia is one of nine battleground states that will decide the Senate majority.

A 43-page document issued by the West Virginia secretary of state titled “Running for office in West Virginia” mentions a “sore loser” or “sour grapes” law that states: “Candidates affiliated with a recognized political party who run for election in a primary election and who lose the nomination cannot change her or his voter registration to a minor party organization/unaffiliated candidate to take advantage of the later filing deadlines and have their name on the subsequent general election ballot.”

“We do stand by the sore loser and sour grapes law,” said Steven Allen Adams, assistant communications director for West Virginia Secretary of State Mac Warner, who is a Republican. Adams said it would be “premature” to discuss future actions they might take, since Blankenship just changed his party registration Monday and has not yet filed all of the necessary paperwork to get on the ballot.

In his statement, Blankenship said, “Though the establishment will likely begin their efforts against us by mounting a legal challenge to my candidacy, we are confident that — if challenged — our legal position will prevail, absent a politically motivated decision by the courts.”