National Republicans believed that Mr. Blankenship would have cost them a winnable Senate seat just as in Alabama last year, when primary voters chose Roy S. Moore, who was accused by several women of making sexual advances toward them as teenagers. In a deeply red state, Mr. Moore lost the general election to Doug Jones, a Democrat.
After more than $1 million in TV attack ads by the McConnell-aligned super PAC last month, polls showed Mr. Blankenship slipping behind two conventional rivals, Mr. Morrisey and Mr. Jenkins. But he surged after a Fox News debate on May 1 — indicative of national interest in the race — in which Mr. Jenkins and Mr. Morrisey mainly attacked each other.
Over the weekend, Mr. Morrisey finally leveled his sights on Mr. Blankenship, calling him a convicted criminal and on Monday sending a letter to his Nevada probation officer, accusing Mr. Blankenship of breaking the law by not filing a financial disclosure.
Mr. Blankenship, raised in poverty in Mingo County, W.Va., now lives principally outside Las Vegas, in a $2.4 million villa, where he is under supervised release from his prison sentence. He failed to file a personal financial disclosure with the Senate Select Committee on Ethics, as required. Mr. Blankenship told The New York Times last month that he felt no pressure to file the disclosure because there isn’t “much of a penalty.”
Mr. Morrisey is a conservative battler who sued the Obama administration repeatedly over environmental regulation. But he will also surely face sharp Democratic attacks: He is a transplant from New Jersey who once worked as a Washington lobbyist for the pharmaceutical industry, running in a state ravaged by the opioid epidemic.
Mr. Manchin, 70, who is one of the most conservative Democrats in the Senate, has managed to retain popularity at home as Democrats elected statewide in West Virginia have become all but extinct. He won his most recent election, in 2012, in a near landslide with 60 percent of the vote even as the Republican presidential nominee, Mitt Romney, carried every county.
In Congress, Mr. Manchin opposed President Barack Obama’s climate change proposals and worked with Republicans on abortion and gun ownership. But he has refused repeated efforts to get him to switch parties, saying that when he grew up, Republicans were the party of the rich, and he did not know any rich people.