WASHINGTON - President Bush is poised to cast the first veto of his presidency today. He'll use it to block legislation that would expand federal research on embryonic stem cells.

Voting 63-37 on Tuesday, the Senate gave final approval to the research, which many scientists think offers the potential for finding cures for Parkinson's, diabetes and other illnesses that afflict millions of Americans.

Now the bill goes to the president. Bush's choice for such a signature event is fraught with potential political consequences. Is it from personal belief or political expedience?

Many Republicans, including Nancy Reagan, California Gov. Arnold Schwarzenegger and North Carolina Sen. Richard Burr, favor the stem cell bill. Most Americans, regardless of political affiliation, support stem cell research.

Although Bush's veto might curry favor with religious conservatives, who compose the cornerstone of his political base, his stand might alienate moderate Republicans, independent voters and others who see the research as key to helping save lives.

"On the Democratic side of things, it's an ace in the hole. It's a good wedge issue and a good base issue that appeals to crossover Republicans," said John Zogby, an independent pollster. "For the president, it's business as usual. This is a guy who's staked his entire career on shoring up the base, and this is a base kind of issue."

But Bush's veto may be not so much a political act as one of personal conscience.

His faith is central to his character, and he has spoken about the issue in the past in moral terms, not electoral ones.

"I believe human life is a sacred gift from our creator," Bush said in an August 2001 nationally televised address on the stem cell issue. "I worry about a culture that devalues life, and I believe as your president I have an obligation to foster and encourage respect for life in America and throughout the world."

Paul Weyrich, chairman and chief executive officer of the conservative Free Congress Research and Education Foundation, said Bush's veto decision is devoid of politics. Weyrich said he remembers imploring Bush to use his veto earlier in his administration but that the president was reluctant to quarrel with a Republican-controlled Congress.

"Finally, there was an issue where he couldn't compromise," Weyrich said. "He's a pro-life president who genuinely believes that destroying an embryo is taking a potential human life."

White House Press Secretary Tony Snow also said the veto is about morality, not politics.

"The president believes strongly that, for the purpose of research, it's inappropriate for the federal government to finance something that many people consider murder. He's one of them," Snow said.

Tony Fabrizio, who was the chief pollster for Republican Bob Dole's 1996 presidential campaign, said Bush's veto could make Republicans in Northeastern and suburban districts more vulnerable.

"This is a great issue in the heartland of Kansas, in Georgia and South Carolina, but does it help you in New York, Connecticut and California?" Fabrizio said. "You can't imagine anyone opposing stem cells would be popular in those districts."