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It's puzzling to me why it's so difficult for some to let go of the old 1990s formulation that the economy still matters most in elections.

"[I]t's really about the economy," declared a BusinessWeek headline yesterday morning. To be sure, the economy has played an important role in the campaign over the last couple weeks. But if Hillary Clinton's victories in Ohio and Texas last night prove anything, isn't it the opposite? Voters are still very much in a Sept. 11th mindset. Clinton won last night in large part by beating Barack Obama two to one among voters who made their decision within the last three days of the race. And she did that by attacking his preparedness to handle national security, not the subprime crisis. Most notably via the now-infamous, and apparently effective, "It's 3:00 A.M...." ad.

Why did that strategy work? Because, as Michael Gerson points out in today's Washington Post, this is really the only issue on which Obama is beatable. Clinton insiders have, it appears, finally grasped this fact. "His vulnerability is experience and judgment on national security," Harold Ickes and Mark Penn wrote in a memo last night.

I suspect foreign policy is now the issue on which Obama's political future will live or die. This morning, he told reporters aboard his campaign plane, "Over the coming weeks we will join [Clinton] in that argument. Was she negotiating treaties? Was she handling crisis? The answer is 'no.'"

John McCain is already signaling that he intends to make November a referendum on national security. Like it or not, it's a foreign-policy election.