President Bush on Monday threatened to veto any legislation that violates the spirit of a proposal he announced last week to give federal funding for limited embryonic stem cell research.

"The statement I laid out (Thursday) is what I think is right for America," Bush told reporters gathered on his central Texas ranch as he signed an agriculture spending bill.

"Any piece of legislation that undermines what I think is right will be vetoed," Bush said.

Later, Bush twice cut off further questions on criticism of his decision. "I answered it Thursday night when I gave an address to the nation," he said. "I spent a lot of time on the subject, I laid out the policy I think is right for America and I'm not going to change my mind."

Bush said his policy was thoroughly thought-out. "It's a moral issue, plus there's a chance that we can save people's lives, and I've laid out the path to do that," he said.

Senators of both parties have said they will try to ease the restriction outlined by Bush when Congress returns next month. Bush aides, including White House chief of staff Andrew Card and Tommy Thompson, the secretary of health and human services, insist the president will stand by his decision, regardless of what scientific breakthroughs may occur down the road.

The comments on Sunday television news shows followed the president's announcement Thursday that he will limit federal funding for embryonic stem cell research to stem cell lines now in existence.

"While it is unethical to end life in medical research, it is ethical to benefit from research where life and death decisions have already been made," Bush wrote in a guest column Sunday in The New York Times.

Appearing on NBC's "Meet the Press," Thompson said the more than 60 embryonic stem cell lines identified by the National Institutes of Health are enough to achieve the basic research needed to continue pursuing cures for juvenile diabetes, Parkinson's, Alzheimer's and other diseases.

He said Bush will stand by his decision regardless of what scientists may discover and estimated that stem cell researchers are three to five years from any breakthroughs.

"This president will not equivocate," Thompson said. "He made a very strong statement on that."

"We think there's more than enough lines for this embryonic stem cell research to go forward," Card added on "Fox News Sunday."

But Sen. Arlen Specter, R-Pa., said he is skeptical those stem cell lines will be enough to find cures.

He said he and Sen. Tom Harkin, D-Iowa, are sponsoring legislation to broaden the federal funding of stem cell research to include discarded embryos from in-vitro fertilization. It will be an issue when Congress considers federal spending legislation next month, Specter said.

"Every day we lose, we're losing lives," he said on CBS' "Face the Nation."

Dr. John Gearhart of the Johns Hopkins University Medical School in Baltimore also questioned whether existing stem cell lines, about half of which are at U.S. laboratories, will be enough.

"We know that there is a shelf life to these, and we are very concerned when we will need more lines, what happens then," Gearhart said on CBS. "And I do think it will be sooner rather than later."

Dr. Leon Kass, a University of Chicago bioethicist heading a Bush-appointed panel monitoring stem cell research, said the existing lines should last at least a decade.

If they prove insufficient, "I think that's a serious question and we will have to revisit it," Kass said on CBS.

Thompson said he couldn't address whether Bush would veto any legislation allowing broader federally financed stem cell research. But he added, "First of all, I don't think the Congress is going to pass that." Lawmakers should let basic research continue before approving such legislation, he said.

Many abortion opponents, including Roman Catholic leaders, say Bush went too far.

Bishop Joseph A. Fiorenza, president of the National Conference of Catholic Bishops, said on ABC's "This Week" that he considered the existing stem cell lines "ill-gotten goods."

"For the government to allow funding for this experiment makes the government complicit in what we consider to be wrongdoing," Fiorenza said.