THE SENATE WILL soon take up a bill -- already passed by the House -- to liberalize the Bush administration's policy on federal funding of stem cell research. Under the current policy, government money can be used to fund research on certain embryonic stem cell colonies -- known as "lines" -- but not on any begun after the policy itself was adopted in 2001. The new bill, pushed by Sens. Arlen Specter (R-Pa.) and Tom Harkin (D-Iowa), would relax this standard considerably, permitting federal support for study of a wide array of stem cell lines generated from embryos left over from in vitro fertilization. President Bush has promised a veto. The Senate should make him do it.

Mr. Bush's policy on stem cell research was not as bankrupt an idea as his fiercer critics sometimes make out. It allowed federal money to begin flowing to a field that might promise dramatic breakthroughs in the treatment of devastating diseases. At this point, however, the policy has outlived its value and is impeding research. Consequently, the public now faces the question of whether to let moral anxiety about the use of human embryos frustrate science that could save and improve many lives.

This might be a difficult choice were these embryos not being created and destroyed anyway. But these small clusters of cells, which are not yet even fetuses, are routinely generated in fertilization clinics in quantities that exceed the number of embryos that will actually be implanted in women. They will never grow into babies; the only question is whether they will be discarded or used in a fashion that benefits humanity.

As the Senate vote draws nearer, there has been talk of alternative legislation that would fund research into promising scientific techniques that might produce stem cell lines without requiring embryo destruction. The idea is to peel off conservative support for liberalizing the president's policy by creating an option that could still be framed as supportive of stem cell research. Alternative strategies for creating stem cell lines -- an idea discussed yesterday in an op-ed column by Leon R. Kass, chairman of the President's Council on Bioethics -- should certainly be explored, and it makes sense for Congress to support such research. But these techniques are, at this stage, nascent and uncertain and have not yet successfully yielded cell lines. They therefore cannot now support the research that is so urgently needed. Federal support for research into their viability is at best a complement to -- not a substitute for -- funding the full range of study possible now on embryonic lines. Such hypothetical alternatives should not be permitted to derail an important change in policy.