KABUL, Afghanistan — Abdullah Abdullah, the front-runner in Afghanistan’s presidential election campaign, announced Sunday that he had won the endorsement of Zalmay Rassoul, the third-place candidate, as part of his effort to gather enough support to win in the next round of voting.

Together the two men’s tickets took about 56 percent of the vote in the first round of voting on April 5, but there is no guarantee that voters would vote the same way in a second round, tentatively set for June 14.

Adding to the prospect that Mr. Rassoul may not bring all his first-round votes with him is that his team appears to have split, with one of his two vice-presidential running mates declining to support Mr. Abdullah’s campaign.

Mr. Abdullah’s camp and some analysts worry that a runoff could be rife with fraud and that there is a considerable risk that it could be disrupted by the Taliban. The insurgents’ campaign of violence failed to have much impact in the first round, but the Taliban could redouble their efforts to intimidate voters in a runoff.