Former Massey CEO and West Virginia Republican Senatorial candidate Don Blankenship. (Steve Helber/AP)

The mainstream wing of the Republican Party is wading deeper into a pair of key Senate races, opposing controversial GOP candidates who could make it harder to hold the majority in the midterms.

The U.S. Chamber of Commerce released its first television commercial of the campaign Friday, a 30-second advertisement touting Sen. Cindy Hyde-Smith (R-Miss.). Hyde-Smith faces a challenge from state Sen. Chris McDaniel, an insurgent conservative and a foe of Senate GOP leaders.

In West Virginia, a Republican-led organization called Mountain Families PAC has filed paperwork to run ads opposing GOP Senate candidate Don Blankenship. The former coal executive has served a one-year prison sentence for conspiring to violate mine safety and health standards after a 2010 underground explosion killed 29 miners.

The advertising investments come as attention in the party has shifted toward preserving control of the Senate, as worries about losing the House majority grow each day.

Republicans are defending a slim 51-to-49 Senate majority in the midterms. Democrats, however, are defending far more seats, including many in states that Trump won, such as West Virginia. On the other hand, GOP officials are on edge about a handful of seats they are trying to hold onto, including the one in Mississippi.

Steven Law, the president of the Senate Leadership Fund, a Republican group, said in a recent interview with Fox News that he was concerned about “the wrong candidate getting nominated” in West Virginia and Mississippi.

The new Mississippi ad casts Hyde-Smith is a “conservative” and highlights her ties to the state’s popular Republican governor, Phil Bryant.

“There’s a reason Governor Bryant chose Cindy to go shake up Washington,” the narrator says. “Because conservative Cindy Hyde-Smith will fight for Mississippi til the cows come home.”

The Chamber of Commerce, the nation’s largest business lobby, said the ad is backed by a six-figure investment.

Hyde-Smith, the former state agriculture and commerce commissioner, was sworn into the Senate this week. Bryant appointed her to the Senate to succeed Republican Thad Cochran, who stepped down this month amid health issues.

Now, Hyde-Smith must run in a November special election if she wants to fill out the remainder of Cochran’s term. McDaniel, who nearly defeated Cochran in a nasty 2014 primary, is also seeking the seat. He has been attacking Hyde-Smith for being a former Democrat.

President Trump and Senate Majority Leader Mitch McConnell (R-Ky.) urged Bryant to appoint himself and run in November. Neither Trump nor McConnell has endorsed Hyde-Smith in the campaign so far.

A pair of Democrats have entered in the race; one is Mike Espy, a former House member who was Mississippi’s first black member of Congress since Reconstruction.

There will be no partisan primaries. If no candidate gets a majority of the vote on Nov. 6, the top two finishers will advance to a runoff. Republican strategists fear that if it comes down to McDaniel and a Democrat, McDaniel’s hard-line views could put the seat in play, even in a heavily conservative state.

In West Virginia, Republicans are trying to unseat Democratic Sen. Joe Manchin III. Like Mississippi, the state voted heavily for Trump in 2016. But the GOP is dealing with a contested Senate primary campaign that will culminate with a vote on May 8.

Federal Communications Commission records, which were first reported by the Charleston Gazette-Mail, show Mountain Families PAC opposes Blankenship and has reserved airtime on West Virginia television.

While the identities of the group’s donors were not clear Friday, the treasurer is Benjamin Ottenhoff, federal campaign finance records show. He has been affiliated with the National Republican Congressional Committee in the past, according to the Center for Responsive Politics.

Republicans are worried a Blankenship primary victory could cost them a chance at the seat. Earlier this month, 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 in West Virginia.

Ottenhoff did not immediately respond to a request for comment Friday.

It is not uncommon for groups with locally themed names and national ties to pop up in key contests. With many voters wary of national interference, there is incentive to try to shield out-of-state involvement.

In last year’s special election for the Senate in Alabama, Democrats started a group called Highway 31, named for the road connecting Alabama’s four major cities. Post-election records showed the national group Senate Majority PAC spent more than $3 million through Highway 31 to help Democratic Doug Jones win.