WASHINGTON — The scene has stayed with me for six years: Democrat Jill Long Thompson, in the midst of a fiercely competitive race in Indiana’s 2nd Congressional District, was being pressed by supporters to criticize what they saw as President Bush’s rush to war in Iraq.

She would have none of it, explaining that her differences with Republican Chris Chocola were on domestic economic issues, not foreign policy. In her district, she said later, “we will support our president, and we will support our troops.”

It was like that all over the country in 2002: Democrats in large numbers ran away from foreign policy or just said “me, too.” Many went down to defeat, including Long Thompson, though last month she won the Democratic nomination for governor.

Things have changed in six years. For one thing, Chocola was voted out in 2006 when frustration over Iraq helped the Democrats sweep to power in the House. Barack Obama is unabashed this year in repeating everywhere he goes that the Iraq war “should never have been authorized and should never have been waged.”

But with economic distress so high, and with John McCain claiming national security as his trump card, Democrats may again be tempted to downplay foreign affairs so they can turn the election into a fight over domestic questions about which McCain has had little to say.

Evading national security, says Sen. Joe Biden, D-Del., would be a disastrous mistake. “The only way we lose this election is not to engage this issue head-on,” Biden said during an interview in his Capitol office the day after Obama clinched the nomination. Democrats, Biden said, should be “proactive” and not “play defense on foreign affairs” because “the case against McCain and Bush on national security is so overwhelming. … It should be an essential part of the case for the Democratic nominee.”

I visited with Biden since he should be at the top of any list of vice presidential picks for Obama. Why Biden? In part because of where he took our discussion: Few Democrats know more about foreign policy, and few would so relish the fight against McCain on international affairs. Few are better placed to argue that withdrawal from Iraq will strengthen rather than weaken the U.S.

The worst thing in a running mate is the fear of muddying his or her image in political combat. Biden would be a happy warrior.

He was born in Scranton, Pa., an essential state for Democrats, and has been a regular in the Philadelphia media market. Pennsylvania Gov. Ed Rendell, himself a plausible No. 2, has called Biden “a perfect fit.” The senator has been through two of his own presidential campaigns in which he experienced what an acquaintance of his called the “white hot heat” of scrutiny.

Biden is Catholic and hails from a blue-collar world, two constituencies with which Obama needs help. The chairman of the Foreign Relations Committee and the former chairman of the Judiciary Committee, Biden speaks with real learning on international affairs and the judiciary — the next vacancies on the Supreme Court should be a big issue in this campaign — while never sounding like an elitist.

But the central reason to pick Biden is the message the choice would send about Obama’s readiness to contest national security issues and his understanding that fixing American foreign policy must be one of the next president’s highest priorities.

Biden has been critical of Bush’s approach to Iraq and the world for the right reasons, and from the beginning. In the fall of 2002, he tried, with Republican Sens. Chuck Hagel and Richard Lugar, to pass a more modest war resolution that put additional constraints on Bush. Then-House Democratic leader Richard Gephardt short-circuited the effort by cutting a deal with the president. Even before the war began, Biden was warning of the costs of a lengthy occupation and predicting a decade-long intervention.

He is also frank about his misunderstanding of what Bush would do. At one point, he thought Bush was reluctant to start a war.

“I vastly underestimated the total incompetence of this crew,” he says. “I could not fathom that they would do what they did under the circumstances they did it.”

To restore its strength and influence, the United States needs to return to the realistic internationalism of FDR, Truman and, yes, the first President Bush. Whether or not Obama picks Biden, he should listen to what Biden is saying. Obama can’t sidestep the foreign policy debate. He has to win it.

E.J. Dionne’s e-mail address is postchat(at)aol.com.