WASHINGTON  In an abrupt shift, Senate Democratic leaders said they would not provide the $80 million that President Obama requested to close the detention center at Guantánamo Bay, Cuba. The move escalates pressure on the president, who on Thursday is scheduled to outline his plans for the 240 terrorism suspects still held there.

In recent days, Mr. Obama has faced growing demands from both parties, but particularly Republicans, to lay out a more detailed road map for closing the Guantánamo prison and to provide assurances that detainees would not end up on American soil, even in maximum security prisons.

The move by Senate Democrats to strip the $80 million from a war-spending bill and the decision to bar, for now, transfer of detainees to the United States, raised the possibility that Mr. Obama’s order to close the camp by Jan. 22, 2010, might have to be changed or delayed.

“Guantánamo makes us less safe,” the Senate majority leader, Harry Reid of Nevada, said at a news conference where he laid out the party’s rationale for its decision, which is expected to be voted on this week. “However, this is neither the time nor the bill to deal with this. Democrats under no circumstances will move forward without a comprehensive, responsible plan from the president. We will never allow terrorists to be released into the United States.”