WASHINGTON -- After almost a year of silence on the issue, the U.S. Senate is on the brink of approving an expansion of federal support for embryonic stem cell research - a measure that divides the Republican Party, pits Congress against the White House and is almost certain to be blocked by the first veto of President Bush's tenure in office.
The issue, scheduled to come before the Senate this week, is forcing Republicans to make a choice between two political constituencies: social conservatives who believe the research is immoral and advocates for patients with debilitating diseases, who have won the sympathy of prominent Republicans as well as Democrats.
The issue is also forcing political candidates to take sides in a debate filled with emotion, religious fervor and scientific nuances.
In Missouri, GOP Sen. Jim Talent - one of his party's most endangered incumbents seeking re-election this year - pleased conservative activists by opposing a ballot initiative that supports embryonic stem cell research.
In Wisconsin, Democratic Gov. Jim Doyle's struggle to win re-election may be helped by his support for stem cell research in a state where it could be big business. House Democrats are trying to use the issue against Republicans in suburban districts, where they believe voters support the research.
The divisions among Republicans will come under the national spotlight this week when the Senate debates legislation to loosen the rules that govern federal funding for stem cell research. The bill, which the House passed last summer, is expected to pass the Senate with the support of prominent Republicans, among them Majority Leader Bill Frist of Tennessee.
But Bush is expected to follow through on his promise to veto the bill.
Some Republicans, especially moderates, fear the veto would reinforce Democrats' argument that the GOP is beholden to the religious right and is an obstacle to scientific progress.
"That's a bad issue to make your first veto," said Rep. Tom Davis, a Virginia Republican.
But for Bush's allies among social conservatives, the expected veto would be a powerful statement of principle.
"For the president, this is really an ethical line that shouldn't be crossed," said David Christensen, director of congressional relations for the conservative Family Research Council.
At issue is research that involves destroying human embryos in the early days of existence to obtain the stem cells inside. Stem cells are thought to develop into any type of cell in the body - a power that many scientists believe may be harnessed to treat diseases, such as Parkinson's, that are incurable.
That hope has prompted many scientists and advocates for patients to call for the federal government to relax current limits on funding for the research.
But Bush and many other conservatives believe that the destruction of embryos is on a par with abortion, and that taxpayers should not finance research they believe is immoral. They also say the potential of embryonic stem cells to treat disease is overstated.
A recent poll released by the Coalition for the Advancement of Medical Research, which backs the legislation, found that 72 percent of Americans support embryonic stem cell research.
Critics dismiss that finding, noting that a poll by the U.S. Conference of Catholic Bishops found far less support for such research when the poll question clearly stated that embryos are destroyed in the process. Asked that way, only 39 percent supported federal funding.
Under current policy, scientists can use federal money for the research only if they work with the limited pool of stem cells that were created before August 2001, when Bush laid out his stem cell policy. Bush said that limitation was aimed at making sure federal funding did not prompt scientists to destroy additional human embryos in order to create stem cells.
The bill approved by the House last year would render more stem cell research eligible for federal aid. It would allow federally funded scientists to work with stem cells created at any time, as long as the cells came from fertility clinic patients' embryos that had been slated for destruction.
The House's 238-194 vote was a watershed, because the House traditionally has been a stronghold for anti-abortion forces. It was a sign that even some conservative abortion foes had come to dismiss the view that destroying an embryo in research was tantamount to abortion.
Another turning point came in the Senate last summer when Frist, in a big blow to his usual conservative allies and to Bush, threw his weight behind the House bill. Ever since, Democrats have criticized Frist for failing to bring the measure to a vote.
Late last month, Frist struck a compromise with conservative critics: He would bring the legislation to the floor, along with two other bills backed by critics of embryonic stem cell research. One aims to encourage ways to accomplish the same purposes as embryonic stem cell research, but without destroying embryos. The other bill would address the fears of some critics that scientists are aiming to create "fetal farms," in which human fetuses might be grown for their organs and tissues. The bill would make it illegal to perform research on tissue from embryos that had come from a human pregnancy initiated for research purposes.
All three bills are considered likely to reach Bush's desk. Bush is expected to veto the bill that would expand embryonic stem cell funding, and sign the other two. That strategy could give political cover to Republicans nervous about appearing to oppose research progress.
Republican leaders are hoping to wrap up the divisive issue, complete with the presidential veto, by the end of the week, a senior GOP aide said.
Janet Hook writes for the Los Angeles Times.