By Cokie Roberts and Steven V. Roberts

President Bush portrays himself as a principled leader who sticks to his guns, and firm values and clear goals are certainly crucial to any successful presidency. Americans gave Bush a second term in large part because he seemed more determined and decisive than John Kerry.

But eight years in office is a long time. A president who never learns or grows runs the risk of sounding rigid and reactionary. Every change of position is not a flip-flop. Sometimes it’s the only intelligent reaction to shifting circumstances.

That brings up the debate over federal funding for stem cell research, and the very different positions taken by two conservative Republicans, President Bush and Sen. Bill Frist.

Frist, a heart surgeon who wants to be president, has looked carefully at the medical facts and concluded that the true “pro-life” position is to back expanded federal funding. As he told the Senate: “We have to work together to allow science to advance.”

Bush takes a theological view, not a scientific one. He remains adamantly committed to a position he formed five years ago: that very limited federal support for stem cell research is sufficient to encourage scientific progress. Never mind that experts like Frist say this view no longer makes sense. Damn the facts, the president has his principles.