WASHINGTON — President Obama signaled his intention yesterday to press forward on his plan to close the Guantanamo Bay prison, despite a growing challenge from Republicans and Democrats, and a limited set of options to make his detainee policy work.

In a sign of how much momentum he has lost, the Senate yesterday voted 90-6 to block funding for the shutdown, a vote taken in a politically charged environment that followed criticism that the administration was backtracking on the security of Americans.

In an effort to retake the political momentum, Obama plans an address today to forcefully defend his proposal for closing Guantanamo by year's end. In the morning speech, at the National Archives in Washington, Obama also will address prospects for a controversial proposal to hold detainees indefinitely without trial, if necessary, and will reassert his argument that closing the prison would advance U.S national security.

"The president signed an order early in his administration to close it, and he intends to keep that promise," said Robert Gibbs, the White House spokesman.

In one possible sign of a new approach, an administration official said that for the first time, a Guantanamo detainee is being sent to the United States to stand trial in a U.S. criminal court. Ahmed Khalfan Ghailani, a Tanzanian captured in Pakistan in 2004, had been indicted by a federal grand jury in New York for participating in attacks on two U.S. embassies in Africa in 1998.

Obama met with leaders of human rights organizations yesterday, as Congress debated the issue and the White House planned its response.

However, following Obama's decision to close the prison four months ago, few new options have emerged to ease the way to a shutdown. To clear the political logjam, the administration and Congress face difficult and politically unpopular policy choices.

"The president is going to have to spend political capital. He will have to lean on people and call out the political cowards," said John Hutson, a retired admiral who advised Obama on detention policy during his presidential campaign. "He is going to have to regain the high ground and the initiative. He had the initiative, and it slipped away."

The Obama administration counterattack began yesterday, when a top Pentagon official countered the growing congressional opposition to moving detainees to the United States, saying the U.S. must place some detainees in mainland prisons.

"This is a case where we need to ask members of Congress to take a more strategic view," said Michele A. Flournoy, the undersecretary of defense for policy. But Republicans seized on remarks yesterday by FBI Director Robert Mueller, who told Congress detainees could pose risks, such as radicalizing others, in U.S. prisons.