After almost a year of silence on the issue, the Senate is on the brink of approving an expansion of federal support for embryonic stem cell research -- a measure that bitterly divides the Republican Party, pits Congress against the White House and is almost certain to be blocked by the first veto of George W. Bush's presidency.
The issue, which is scheduled to come before the Senate this week, is forcing Republicans to make a dicey choice between two potent political constituencies: social conservatives who believe the research is immoral, and advocates for patients with debilitating diseases, who have won the support of prominent Republicans as well as Democrats.
The issue is also forcing political candidates nationwide to take sides in a debate that is shot through with emotion, religious fervor and scientific nuances.
In Missouri, Sen. Jim Talent -- one of the GOP's most endangered incumbents seeking reelection this year -- pleased conservative activists by opposing a ballot initiative that supports stem cell research.
In Wisconsin, Democratic Gov. James Doyle's reelection bid may be helped by his support for stem cell research in a state where it could be big business.
Across the country, House Democrats are trying to use the issue against Republicans in suburban districts, where they believe voters tend to support the research.
The divisions among Republicans will be in the national spotlight this week when the Senate debates legislation that would loosen restrictions on federal funding for embryonic stem cell research.
The bill, which the House passed last year, 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 quickly follow through on his promise to veto the bill.
Some Republicans, especially moderates, fear a veto would reinforce Democrats' argument that the GOP is beholden to the religious right and is obstructing scientific progress.
"That's a bad issue to make your first veto," Rep. Thomas M. Davis III (R-Va.) said.
But for many social conservatives, the 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 Family Research Council, a conservative advocacy group.
At issue is research that involves destroying human embryos to obtain stem cells, which are thought to be able to develop into any type of cell in the body. Many scientists believe such research may lead to cures for diseases such as Parkinson's and Alzheimer's.
That hope has prompted many scientists and patients' advocates to call for looser limits on federal funding for the research.
But Bush and many other conservatives liken the destruction of embryos to abortion, and say that taxpayers should not finance what critics call immoral research. 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% of Americans supported embryonic stem cell research -- a finding that supporters say is a measure, in part, of how many lives have been touched by the illnesses that such research might help cure.
Critics dismiss that finding, noting that a poll released by the U.S. Conference of Catholic Bishops found that when the poll question clearly stated that human embryos were destroyed, 39% supported federal funding for such research.
Under current law, scientists can use federal money for the research only with lines of stem cells 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 to obtain 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 would otherwise be destroyed.
The House's 238-194 vote was a watershed, because the House traditionally has been a stronghold for antiabortion 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 when Frist, in a blow to his usual conservative allies and to Bush, last summer threw his weight behind the House bill. But ever since, Democrats have criticized Frist for not bringing the measure to a vote.
Late last month, Frist struck a compromise with his conservative critics: He would bring the legislation to the floor, but along with two other bills backed by critics of embryonic stem cell research. One would encourage ways to accomplish the purposes of embryonic stem cell research 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 would be grown for their organs and tissues. The bill would make it illegal to perform research on fetal tissue gestated 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.
"They want to move this along quickly for political reasons," said Rep. Michael N. Castle (R-Del.), principal author of the bill Bush would probably veto. "They understand there is huge acceptance of this," Castle said, referring to embryonic stem cell research, "and they are just appealing to a small minority in opposition."
Castle said he would work to build support for overriding Bush's expected veto, but he acknowledged that was a long shot. It would take a two-thirds majority in the House and Senate to override the veto; the House vote for the bill last year fell short of that margin.
A veto would kill the issue for the rest of the year in Congress, but the debate will continue in political campaigns scattered across the country.
The issue is front and center in Missouri, where the state ballot will include an initiative to prevent the state from banning any stem cell research allowed under federal law. The initiative is opposed by social conservatives and groups such as Missouri Right to Life, but it is backed by another important GOP constituency -- business leaders -- as well as former Sen. John C. Danforth (R-Mo.).
When Talent, who is facing a tough Senate reelection challenge in the state, came out in opposition to the ballot measure, it helped him win back favor among a politically important bloc of conservatives. Early this year, he angered some conservatives when he dropped his support for an anti-cloning bill, which he believed would go too far and outlaw an alternative research method he supported.
Patty Skain, executive director of Missouri Right to Life, said her members were "definitely more comfortable with him now" that Talent was opposing the ballot initiative.
Democrats are hoping that the ballot initiative will increase turnout among supporters of Talent's Democratic opponent, State Auditor Claire McCaskill, who backs the measure.
In Wisconsin, where human embryonic stem cells were first isolated, Gov. Doyle is in a tough fight for reelection against Rep. Mark Green (R-Wis.).
Although the race will hinge mostly on other issues, such as a state corruption scandal, the two candidates have starkly different positions on embryonic stem cell research: Doyle is for it, Green is against it.
Jennifer Duffy, an election analyst for the nonpartisan Cook Political Report, said that distinction was more likely to work in Doyle's favor.
"It's a vote motivator, and it splits the Republican base," Duffy said.
In House races, the Democratic Congressional Campaign Committee is raising the issue in some suburban districts where strategists believe Republicans will suffer if they voted last year against expanding embryonic stem cell research. The committee early this year launched Web advertisements on the issue against seven Republicans.
"On one side of the divide is life-saving medical research. On the other side is special-interest politics," said Rep. Rahm Emanuel (D-Ill.), chairman of the committee.
But Brian Nick, a spokesman for the National Republican Senatorial Committee, said he doubted that many voters would decide between candidates based on their views on stem cell research.
"Does it truly move a large number of people either way?" he asked. "I've not seen anything like that in this election cycle."