WASHINGTON — For Lawrence H. Summers, President Obama’s preferred candidate to lead the Federal Reserve, the messy debate over a military attack in Syria was the final sign.

After weeks of opposition to his candidacy from an array of progressives, the president’s inability to rally Congressional Democrats on Syria persuaded Mr. Summers that his most important audience — the Senate, which must confirm a Fed chairman — probably could not be won over.

He concluded that the White House was also unlikely to overcome opposition to his candidacy from many of the same Democrats, who view him as an opponent of stronger financial regulation, according to supporters who insisted on anonymity to describe confidential conversations with him.

“Clearly Obama couldn’t bring his own most enthusiastic supporters to back him on an issue of national security,” one supporter said. “How was he going to corral them for Larry?”