WASHINGTON — Michelle Obama bristled at charges that her husband lacked enough experience for the presidency.

“We’ve heard this spewed from the lips of rivals every phase of our journey: He is not experienced enough, he should wait his turn,” she recently told supporters of Senator Obama, a Democrat of Illinois, who at 45 is serving his first Senate term. Only political insiders, she said, would look at his life accomplishments “and dare to have the audacity to say he is not ready.”

Experience — and how to measure it — has become one of the first big debating points of the 2008 presidential race.

In one of the curiosities of the Democratic primary, some of the candidates with the most experience in national politics are at the bottom of the early popularity surveys. By contrast, Mr. Obama, with a mere three years on the national stage, is this year’s campaign-trail sensation.

And so Senator Dodd of Connecticut (with 33 years in the House and Senate) has been trying to heighten the importance of Washington knowledge, making a constant refrain of his claim that President Bush proves the dangers of on-the-job training in the White House. “I think people do care about experience,” Mr. Dodd said.

Governor Richardson of New Mexico (15 years in the House, two years as U.N. ambassador, three years as Energy secretary) touts his “unparalleled experience.” And Senator Biden (35 years in the Senate) has said of his campaign rivals: “It’s not so much whether I can compete with their money, but whether they can compete with my ideas and my experience.”

Even a former senator of North Carolina, John Edwards, who served a single term before opening a White House bid in 2004, has brought his twist to the issue. Asked at an event last month how he differed from Mr. Obama, Mr. Edwards said: “Experience. I’ve been through a presidential campaign.”

Advocates for Mr. Obama, as for other candidates who are positioning themselves as outsiders to Washington’s political culture, like to say that the range of their life experiences makes them more fit for office than those who have spent their careers in government. In Mr. Obama’s case, that résumé includes stints as a community organizer, law professor, civil rights attorney, and eight-year member of the Illinois state Senate.

Mr. Obama’s allies also assert that a wealth of government experience did not make Vice President Cheney or a former defense secretary, Donald Rumsfeld, any wiser in confronting the nation’s problems. And Mr. Obama launched his campaign from Springfield, Ill. — a city identified with Abraham Lincoln — perhaps to remind voters that one of the country’s greatest presidents had little Washington experience before he reached the White House. (Lincoln had served one House term and 12 years in the Illinois Legislature.)

Among conservative voters, experience is prized. “Republicans generally believe, particularly conservatives, that we are electing a wartime president, so experience will be critical,” GOP political consultant Christopher Barron said. “It’s one of the reasons you continue to see someone like Rudy Giuliani over-performing among conservatives who disagree with him on a litany of social issues.”

Mr. Giuliani, who is a former New York City mayor and federal prosecutor, built a national reputation as a steady leader after the September 11, 2001, attacks. He leads in several early polls of Republican voters.