WASHINGTON  Ever since the G.I.’s came home from World War II, it has been the nation’s policy to reward war veterans with college education. Now, a bipartisan proposal to expand that benefit significantly for today’s veterans has encountered a new complication: the military still needs its fighting men and women in uniform, not in classrooms.

With the wars in Iraq and Afghanistan far from over, President Bush is threatening to veto a bill that would pay tuition and other expenses at a four-year public university for anyone who has served in the military for at least three years since the attacks of Sept. 11, 2001. A main reason is the fear that it would hasten an exodus from the ranks.

The issue has created a political conundrum for a president who has often gone to great lengths to show support for the troops. And in an election year, when legislative battles on Capitol Hill are increasingly turning into proxy fights of the presidential campaign, that is almost certainly the point.

“I would say the president really has a choice here,” Senator Jim Webb, a Virginia Democrat who has led the Senate’s efforts to expand the benefits, said Sunday on “Meet the Press.” Mr. Bush, he said, needed “to show how much he values military service.”