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Mr. Loomis said, “Roberts is receiving substantial support from almost all Kansas Republicans. I don’t see much of an opening at all for Wolf, even if some outside groups may give him some assistance.”

Vulnerable incumbents

Jennifer Duffy, of the Cook Political Report, said there is room for Mr. Wolf to outflank Mr. Roberts on the right. “These days, I don’t think any Republican incumbent can take a primary challenge lightly, and Roberts is no exception,” she said.

Al Cardenas, chairman of the American Conservative Union, said Mr. Roberts‘ 86 percent lifetime rating with his group is “satisfactory” and enough to win its support in the Republican primary.

“Some of our friends out there are getting a bit out of hand,” Mr. Cardenas said. “My sense is it’s people who have more of a personal agenda than political agenda.”

He also likened the attacks against Mr. Roberts to the attempts to cast Sen. John Cornyn as a liberal ahead of his re-election effort in Texas.

“How can anyone primary him and call him a liberal? It is a joke, and it detracts from the credibility of the [tea party] movement,” he said. “I plan on supporting John Cornyn and Pat Roberts.”

Mr. Roberts is ranked as the fifth most conservative member of the Senate in the most recent breakdown from Heritage Action, the political arm of the Heritage Foundation.

Election-year shifts?

Mr. Roberts‘ opponents say voters shouldn’t be fooled by his voting record this year and point out that he ranked much lower on Heritage Action’s 2012 list, with a score of 65 percent.

They also accuse him of adopting the same strategy that Sen. Orrin G. Hatch, Utah Republican, employed during his re-election fight, running to the right during the campaign only to return to his moderate ways after he won another six-year term.

“It’s important to remember that some of these incumbents are not liberals or even moderates,” Daniel Horowitz, the group’s policy director, said on they website. “But they are not conservatives either. They are a ruling class of special-interest career politicians who pursue personal power as an ends to itself. When it suits their need to cast some conservative votes, they will do so. But when they need to placate the special interests, they will jump in head first. They certainly will never put their careers on the line to fight for us.”