WASHINGTON — Each morning for the last week, at 7:45, more than a dozen White House aides have mustered in the corner office of President Obama’s chief of staff, Denis R. McDonough, to get their marching orders for what has become the most intense, uphill lobbying campaign of the Obama presidency.

The White House’s goal is to persuade Congress to authorize a limited military strike against Syria to punish it for a deadly chemical weapons attack. But after a frenetic week of wall-to-wall intelligence briefings, dozens of phone calls and hours of hearings with senior members of Mr. Obama’s war council, more and more lawmakers, Republican and Democrat, are lining up to vote against the president.

Officials are guardedly optimistic about the Senate, but the blows keep coming. On Saturday, Senator Mark Pryor, Democrat of Arkansas, perhaps the most endangered incumbent up for re-election, came out against the authorization to use force.

In the House, the number of rank-and-file members who have declared that they will oppose or are leaning against military action is approaching 218, the point of no return for the White House. Getting them to reverse their positions will be extremely difficult.