Washington (CNN) The Republicans' bid to keep or expand their majority in the Senate starts Tuesday in West Virginia, Indiana and Ohio, with hotly contested primaries to take on three of the nation's most endangered Democratic incumbents.

The political world will be watching West Virginia to see if Don Blankenship -- the ex-convict coal baron whose late surge has Senate Majority Leader Mitch McConnell's allies fretting that he could spoil the party's chances of unseating Democratic Sen. Joe Manchin there this fall -- defeats Attorney General Patrick Morrisey and Rep. Evan Jenkins.

"Let's just hope and pray that doesn't happen," South Dakota's Sen. John Thune, the No. 3-ranking Senate Republican, told reporters after a meeting in McConnell's office Monday afternoon.

"It wouldn't be good," he added.

In Indiana, self-funding former state Rep. Mike Braun, a shirtsleeve-wearing businessman, takes on the two men his campaign ads have cast as cardboard cutouts: Reps. Todd Rokita and Luke Messer.

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