Don Blankenship, a Republican candidate for U.S. Senate, speaks at a town hall at West Virginia University on March 1 in Morgantown, W.Va. Blankenship is the former chief executive of the Massey Energy, where an explosion in the Upper Big Branch coal mine killed 29 men in 2010. (Spencer Platt/Getty Images)

A long-running fight between mainstream Republican leaders and controversial GOP candidates is intensifying in a pair of key Senate races, with contenders lashing out at Senate Majority Leader Mitch McConnell (R-Ky.) as his allies try to elevate their Republican opponents.

In West Virginia, the polarizing former coal-executive candidate Don Blankenship issued a written statement excoriating McConnell on Monday. He called the Senate leader a “Swamp captain” and likened his strategy to Russian involvement in the 2016 presidential election.

“The Russians and McConnell should both stop interfering with elections outside their jurisdictions,” Blankenship said.

The statement came after a group called Mountain Families PAC launched an ad campaign opposing Blankenship. Party leaders fear that if Blankenship is nominated, his past would hurt GOP chances of picking up a crucial Democratic seat. Blankenship has served a one-year prison sentence for conspiring to violate mine safety and health standards after a 2010 underground explosion killed 29 miners.

While Mountain Families PAC’s donors are not yet known, the organization has paid a trio of political firms with ties to McConnell allies, according to a federal campaign-finance filing. Those firms have previously worked with the Senate Leadership Fund, a super PAC helmed by a top former McConnell aide, and the National Republican Senatorial Committee, which is the Senate Republican campaign arm.

McConnell said Tuesday that he doesn’t “pay a whole lot of attention to these primaries,” but sent a clear signal about how much he prizes electability in November.

“We’ll wait and see who the nominee is and get behind a Republican candidate,” McConnell said. “And hopefully it’ll be one who’s actually electable.”

Behind the scenes, McConnell’s close confidants have been closely monitoring primaries in West Virginia and elsewhere. The White House has also taken an interest in the West Virginia race, where Democratic Sen. Joe Manchin III is trying to hold his seat in a state that voted overwhelmingly for Donald Trump in the 2016 presidential race. On a recent visit to the state, Trump snubbed Blankenship, sitting next to his two Republican opponents — Rep. Evan Jenkins and state Attorney General Patrick Morrisey — at a roundtable on tax reform.

The competition between Morrisey and Jenkins is heating up ahead of the May 8 primary. The attorney general released a new ad Tuesday attacking Jenkins as too liberal.

West Virginia is one of 10 states Trump won where Democrats are defending Senate seats this year. GOP leaders are hopeful about making gains in those states to help them hold or expand their narrow 51-to-49 Senate majority in November.

But they must also defend seats that Democrats are contesting. In Mississippi, insurgent conservative Chris McDaniel is challenging recently appointed Sen. Cindy Hyde-Smith. Those two Republicans are joined by two Democrats in an unpredictable race that would go to a runoff if no candidate receives a majority of the vote in a Nov. 6 special election.

The U.S. Chamber of Commerce launched a television ad last week touting Hyde-Smith, marking its first step onto the airwaves in the midterms. Trump and McConnell have yet to endorse Hyde-Smith. But she is seen among many party leaders as preferable to McDaniel, who has been openly critical of McConnell and Senate GOP leaders.

On Twitter this week, McDaniel singled out McConnell, labeling Hyde-Smith his “handpicked” candidate. In fact, McConnell and Trump wanted Republican Gov. Phil Bryant to appoint himself to replace the ailing Thad Cochran and then run for the seat. Bryant declined and has enthusiastically supported Hyde-Smith.

McConnell allies have said they do not believe attacks against him will resonate with Republican primary voters.

Allies of Hyde-Smith and McDaniel have already been positioning to cast their favored candidates as the strongest competitors against the Democrats. The Senate Leadership Fund promoted a poll on Tuesday showing Hyde-Smith in close competition with Democrat Mike Espy and McDaniel lagging behind.

McDaniel released a statement accusing the GOP establishment of trying to create a “false narrative” with the survey.