But for the moment, virtually all the 11 incumbent Republicans now running for reelection appear safe from primary challenges, including more moderate senators like Susan Collins of Maine.

Of the sitting GOP senators, Graham is seen as the most vulnerable to a primary threat, but even he seems on solid ground. The only GOP foe to have emerged is longshot businessman Richard Cash, who owns a fleet of ice-cream trucks. If others get into the race, it could be harder for Graham’s most outspoken foes — largely from the libertarian wing of the party — to unite behind a single candidate.

“I’m a Ronald Reagan Republican, not a Ron Paul Republican,” Graham said.

The lack of tea party challengers has significant implications in the national fight for control of the narrowly divided Senate. If a GOP senator were to lose the primary, Democratic chances of winning that seat would improve dramatically and help the party keep control of the chamber.

The early positioning also affects Senate business. Democrats believe that with Republicans, particularly the top two Senate GOP leaders, worried about conservative primary threats, compromises over issues ranging from the budget to gun control are exceedingly unlikely.