The GOP’s Don Blankenship nightmare isn’t over after all. The West Virginia coal baron announced Monday that he plans to wage a third-party campaign for U.S. Senate after failing to win the Republican nomination earlier this month. While it’s unclear if he can do so under the state’s electoral laws, such a campaign would be a major boost to Democratic Sen. Joe Manchin, and a major blow to Senate Republicans, who launched a desperate bid to stop Blankenship from becoming their nominee.

Blankenship says he now plans to run as the nominee of the Constitution Party. But to do so, he’ll first need to find a way around the state’s so-called sore loser law that bars a candidate who comes up short in a major-party primary from then switching to a minor party for the general election. In his announcement, Blankenship hinted he’d challenge that law in court and, as is his way, suggested the only thing that could stop him was a political conspiracy from his enemies.

“Although 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,” said Blankenship, who recently spent a year in prison for his role in a deadly mine disaster. Blankenship claims he was wrongly convicted in that case and held as a “political prisoner” by the Obama administration.

Senate Republicans feared, not unreasonably, that if Blankenship became their nominee, he’d have spoiled their chances of defeating Manchin, in a state Trump won by more than 40 percentage points. The GOP had originally hoped to ignore the ex-con, and hoped that one of his less controversial opponents—Rep. Evan Jenkins or Attorney General Patrick Morrisey—would carry the day. But Blankenship emerged as a serious contender, despite his criminal record, by casting himself as a truth-telling outsider willing to take on the Washington establishment. His most frequent target was Senate Majority Leader Mitch McConnell, who he attacked in increasingly outrageous ways, including by suggesting he was some sort of drug kingpin and by taking blatantly xenophobic swipes at his Taiwanese-born wife, Transportation Secretary Elaine Chao. McConnell’s allies responded with a covert counterattack, and President Trump ultimately got involved as well.

Blankenship went on to finish a distant third in this month’s GOP primary, losing to Morrisey by about 15 points. But Blankenship made it immediately clear that he had no interest in getting behind the GOP nominee. In an open letter to President Trump after the primary, Blankenship warned Morissey would lose in November, and his campaign manager likewise branded him a “corrupt carpetbagger.” Trump, who had publicly urged his supporters not to vote for Blankenship, reportedly called him after the primary to smooth things over. That effort now appears not to have worked.

If Blankenship’s name does end up on the November ballot, he’d be expected to siphon off a small but potentially significant number of votes from Morrisey. Blankenship won just under 20 percent of votes in the primary, but he has a nearly unlimited amount of money to continue attacking McConnell and Morrisey, in a race where even a few percentage points could be the difference. In short, then, Blankenship could still ruin the GOP’s chances in an otherwise winnable Senate race. Indeed, he seems almost determined to do just that.