WASHINGTON, May 24 - The House passed a bill on Tuesday to expand federal financing for embryonic stem cell research, defying a veto threat from President Bush, who appeared at the White House with babies and toddlers born of test-tube embryos and warned the measure "would take us across a critical ethical line."

Gerald Herbert/Associated Press

President Bush appeared at the White House with babies and toddlers born of test-tube embryos, some wearing shirts that read "former embryo."

The vote, 238 to 194 with 50 Republicans in favor, fell far short of the two-thirds majority required to overturn a presidential veto, setting up a possible showdown between Congress and Mr. Bush, who has never exercised his veto power. An identical bill has broad bipartisan support in the Senate; moments after the House vote, the Senate sponsors wrote to the Republican leader, Bill Frist, urging him to put it on the agenda.

The House action is the first vote on embryonic stem cell research since August 2001, when Mr. Bush opened the door to taxpayer financing for the studies, but only with strict limits. The new bill permits the government to pay for studies involving human embryos that are in frozen storage at fertility clinics, so long as couples conceiving the embryos certified that they had made a decision to discard them.

"The White House cannot ignore this vote," said the bill's chief Republican backer, Representative Michael N. Castle of Delaware, adding, "I'm elated."

But opponents also said they were elated. Representative Joseph R. Pitts, Republican of Pennsylvania, said: "I hate to lose, but I feel pretty good about this vote. We beat a veto-proof margin by 50 votes."

The big question now is what will happen in the Senate. Dr. Frist, a heart surgeon from Tennessee who supports the existing policy, is already facing intense pressure from conservatives over the issue of Mr. Bush's judicial nominees and does not seem eager to schedule a vote on stem cell research. He said last week that he wanted to check with his colleagues before doing so.

The House vote followed an impassioned lobbying campaign by advocates for patients, including Nancy Reagan. Mrs. Reagan, who became a strong backer of stem cell research as her husband struggled with Alzheimer's disease, telephoned fellow Republicans this week urging a yes vote, Mr. Castle said.

But Mr. Bush countered with a powerful one-two punch, throwing the full weight of the White House behind the opposition. On Friday, he issued a rare threat to veto the Castle bill. On Tuesday, just hours before the vote, he appeared in the East Room of the White House with families created by a rare but growing practice in which one couple donates its frozen embryos to another.

"The children here today remind us that there is no such thing as a spare embryo," Mr. Bush said, amid the squeals and coos of babies cradled in their mothers' arms. "Every embryo is unique and genetically complete, like every other human being. And each of us started out our life this way. These lives are not raw material to be exploited, but gifts."

The parents, who worked through a Christian adoption agency, applauded enthusiastically. When Mr. Bush said that "every human life is a precious gift of matchless love," a mother behind him on stage mouthed the word "Amen."

The White House event, on what conservative Christians and the president call an important "culture of life" issue, demonstrated just how far Mr. Bush is willing to assert himself on policy that goes to what he considers the moral heart of his presidency. In another sign of how important the issue is to conservatives, the House Republican leader, Tom DeLay of Texas, managed the opposition to the bill, also casting it in stark moral terms.

"An embryo is a person, a distinct internally directed, self-integrating human organism," Mr. DeLay said, adding, "We were all at one time embryos ourselves. So was Abraham. So was Muhammad. So was Jesus of Nazareth."

He went on: "The choice to protect a human embryo from federally funded destruction is not, ultimately, about the human embryo. It is about us, and our rejection of the treacherous notion that while all human lives are sacred, some are more sacred than others."