Abdullah Abdullah, a longtime opponent of President Hamid Karzai and an ardent supporter of the United States, emerged Saturday as the clear front-runner in Afghanistan’s presidential election.

In preliminary results released Saturday, Mr. Abdullah had won 45 percent of the vote, not enough to avoid a runoff with Ashraf Ghani, a former World Bank economist and Karzai adviser, who had won 32 percent. But Afghan government officials say Mr. Abdullah is on the verge of forging alliances with at least two of the runners-up to gain their support, and possibly the presidency, in the next round.

Either of the top two candidates would represent a significant break with the years of deteriorating relations the United States has had with Afghanistan under Mr. Karzai, and a shift toward greater bilateral cooperation. Each candidate has said, for instance, that he would sign a security agreement allowing American forces to remain in the country past 2014, which Mr. Karzai negotiated but refused to sign.