Sen. Thad Cochran (R-Miss.). (AP Photo/The Clarion-Ledger, Joe Ellis)

The antitax Club for Growth on Wednesday called on Sen. Thad Cochran (R-Miss.) to end his campaign a day after a too-close-to-call primary against tea party-backed Chris McDaniel that could be headed to a runoff.

“Yesterday’s historic vote makes it clear that Mississippians are ready to turn the page to a new generation of bold, conservative leadership. Senator Cochran has served honorably, but the rationale for his candidacy ended yesterday. He said he didn’t want to run again, but everyone asked him to. Well, a plurality of Mississippi Republican voters just proved that they don’t want him to,” said Club for Growth President Chris Chocola in a statement.

He added that Cochran "should do the honorable thing and decline to contest the runoff." Should he choose to continue his campaign, Chocola said, his group "will vigorously pursue this race to its conclusion."

The Club for Growth backs McDaniel and has spent more money supporting him than any other group.

With more than 99 percent of the vote tallied Wednesday morning, McDaniel was ahead by less than one percentage point. McDaniel had 49.6 percent of the vote to Cochran’s 48.9 percent, with a third candidate at under 2 percent. If no candidate can claim more than 50 percent of the vote, it would mean a second faceoff, on June 24.

Cochran did not address supporters Tuesday evening. His spokesman, Jordan Russell, told Post Politics early Wednesday morning, "We expect a runoff."

McDaniel appeared prepared for runoff, too, tweeting late Tuesday, “Whether it’s tomorrow, or whether it’s three weeks from now, we WILL stand victorious.”

Henry Barbour, who runs the pro-Cochran super PAC Mississippi Conservatives, called the Club for Growth's call for Cochran to bow out "pathetic."

"Those groups like the Club For Growth that think they can just ignore the will of Mississippi voters and not have a runoff -- how obnoxious is that?" he asked. Barbour said his group would continue to remain "heavily engaged" in the race on behalf of Cochran.

Updated at 10:53 a.m.