The decision was clearly a hard one for Mr. Abdullah. He choked up at the moment of announcing it before his supporters and had to pause to drink water before speaking.

“It did not come easily,” he told the crowd, which had begun cheering at his announcement. He said people had died in the cause of establishing a democracy and a transparent electoral system that had now become threatened.

Secretary of State Hillary Rodham Clinton, traveling in Morocco, released a statement saying that while the Obama administration would support Mr. Karzai as president, she hoped Mr. Abdullah would “stay engaged in the national dialogue and work on behalf of the security and prosperity of the people of Afghanistan.”

Mr. Abdullah rejected any suggestion of joining Mr. Karzai’s government, and he clearly signaled that he was positioning himself as a future player in Afghan politics. In a news briefing later at his home, he said: “I did it with a lot of pain, but at the same time with a lot of hopes towards the future. Because this will not be the end of anything, this will be a new beginning.”

Mr. Abdullah has been under intense pressure from Western officials to avoid confrontation and end a two-month dispute over the election results. That has been in part because the outcome of the runoff had been identified as a vital benchmark before Mr. Obama was to announce his military strategy in Afghanistan.

Even now, administration officials said Sunday, that announcement could be at least three weeks away: it is not expected until the president returns from a trip to Asia later this month.

Mr. Obama is scheduled to hold at least two Afghanistan meetings at the White House this week, following his session on Friday with the Joint Chiefs of Staff, in which he pushed his military commanders to return with more specific options.