The next battle in the US political war over stem-cell research is looming, after Bill Frist, the top Republican in the Senate, detonated a political time bomb as lawmakers sloped away on vacation. 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 Bush's second term.