Mr. Frist, a heart-lung transplant surgeon who said last month that he did not back expanding financing "at this juncture," announced his decision this morning in a lengthy Senate speech. He said that while he had reservations about altering Mr. Bush's four-year-old policy, which placed strict limits on taxpayer financing for the work, he supports the bill nonetheless.

"While human embryonic stem cell research is still at a very early stage, the limitations put in place in 2001 will, over time, slow our ability to bring potential new treatments for certain diseases," Mr. Frist said. "Therefore, I believe the president's policy should be modified." His speech received the approval of Democrats as well as Republicans.

"I admire the majority leader for doing this," Senator Harry Reid, the minority leader and Democrat of Nevada, said immediately after the speech. He and Senator Dick Durbin, Democrat of Illinois, said Mr. Frist's stance would give hope to people everywhere. Senator Arlen Specter, Republican of Pennsylvania, contending they were discussing "the difference between life and death," said of Mr. Frist, "I believe the speech that he has just made on the Senate floor is the most important speech made this year, and perhaps the most important speech made in years."

He added: "This is a speech that will reverberate around the world, including at the White House."