Frist called for increased federal funding for research on frozen embryos left over from fertility treatments, setting a collision course with President George W Bush.
His move thrust the stem-cell issue back into the mix of political firefights, including those over Iraq, a brewing White House scandal and supreme court appointments, which will frame Bushs second term.
Frist triggered an immediate shockwave among Bushs most conservative supporters, and put the President squarely on the spot, because as Senate majority leader, he has the power to move the legislation on the issue to a vote.
Bush has pledged to wield his first veto if the Bill expanding federal aid for research, which supporters say could help find cures for Parkinsons disease, alzheimers and some cancers, hits his desk. The President is someone who believes we shouldnt be creating life for the sole purpose of destroying it, said Bushs spokesman.
After weeks of agonizing, Bush ruled in August 2001 that federal money could only be used for research on embryos that already existed at that time - ruling out the use of fertilized eggs left over from fertility treatments.
He has since stuck by that policy, despite complaints from scientists, who say they are being left behind the rest of the world, and Bushs political foes, who hammered stem-cell research on the campaign trail last year.