Even so, Mr. Obama has faced growing demands in recent days, from both parties but particularly from Republicans, to spell out in detail how he plans to close the Guantánamo detention center and to provide assurances that detainees would not end up on American soil, not even in maximum security prisons.

The move by Senate Democrats to bar, for now, any transfer of detainees to the United States, raised the possibility that Mr. Obama’s order to close the camp by Jan. 22, 2010, may have to be changed or delayed.

“Guantánamo makes us less safe,” the Senate majority leader, Harry Reid of Nevada, said on Tuesday at a news conference where he laid out the party’s rationale for its decision. “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.”

Senate Democrats said they still backed Mr. Obama’s decision to close the prison. But lawmakers have not exactly been eager to accept detainees in their home states. When the tiny town of Hardin, Mont., offered to put the terrorism suspects in its empty jail, Montana’s senators, both Democrats, and its representative, a Republican, quickly voiced opposition.

Administration officials have indicated that if the Guantánamo camp closes as scheduled more than 100 prisoners may need to be moved to the United States, including 50 to 100 who have been described as too dangerous to release.

Of the 240 detainees, 30 have been cleared for release. Some are likely to be transferred to foreign countries, though other governments have been reluctant to take them. Britain and France have each accepted one former detainee. And while as many as 80 of the detainees will be prosecuted, it remains unclear what will happen to those who are convicted and sentenced to prison.

At the White House on Tuesday, the press secretary, Robert Gibbs, said the administration expected that Congress would eventually release the money to close the camp, and he suggested that the concerns of lawmakers would start to be addressed on Thursday, when Mr. Obama will present a “hefty part” of his plan.