Four years ago, Senator Bill Frist, speaking as a physician and former medical researcher, took the lead in the debate over embryonic stem cell research by urging lawmakers not to limit this “promising and important line of inquiry” with unscientific restrictions. Now, as the issue approaches a climactic moment, Mr. Frist, speaking as the Republican majority leader and someone with presidential ambitions, will lead the debate as a convert to President Bush’s unrealistic hobbling of full-fledged research financing. The results will affect the fate of a worthy and overdue commitment to add funds for greater research, something that has already been approved by the House.