WASHINGTON – Barack Obama knows it’s a stretch to think of him as president.

Just 46 years old and three years out of the Illinois legislature, the freshman senator also knows that the clock is ticking on his chance to surmount that “certain threshold” and convince voters he’s ready for the White House.

“The challenge for us is to let people know what I’ve accomplished at a time when the campaign schedule is getting so compressed,” Obama said in a recent interview. “I just don’t have much time to make that case.”

He’s right about that. Iowa Democrats begin winnowing the field late this year or in early January with their first-in-the-nation caucuses.

Sen. Hillary Rodham Clinton and 2004 Democratic vice presidential nominee John Edwards are tied with Obama in polls of Iowa Democrats. The former first lady has a huge lead in national surveys of Democratic voters, and the backing of a political machine built by her husband, former President Clinton.

Obama has the broadest network of grass-roots activists, or at least that seems to be the case based on the record number of people who have donated money to his campaign – often in small amounts – and the size of crowds at his campaign events.

But he’s got that nagging problem …

“… People have to feel comfortable that, ‘You know what? This guy can handle the job,'” he said between campaign stops last weekend in Iowa.

“It’s a stretch for them because I haven’t been on the national scene for long and haven’t gone through the conventional paths that we traditionally draw for our presidents, so they’ve got to stretch a little bit during a period where there’s a lot of stuff going on internationally, right?” said the unusually self-aware Obama.

Obama’s rivals, especially Clinton, don’t want voters making that leap of faith.

They pounce on Obama’s every gaffe (i.e. referring to U.S. lives lost in Iraq as “wasted”), exploit any misstatement (saying 10,000 people died in a tornado that actually killed 12) and call Obama naive for stating the obvious (nuclear arms against Afghanistan and Pakistan are not an option).

The first-term Illinois senator hasn’t helped his case with a string of shaky debate appearances, a streak he ended Sunday with a strong performance in Iowa as his more experienced rivals took aim.

Obama hopes he still has time to win the job.

“I think it’s fair that I’ve got to earn the confidence of the electorate,” Obama told AP. “What we’ve tried to do over the course of the last six months is make the case for change, and the American people are desperately hungry for change. The next four or five or six months will involve me making the case that not only am I the most effective change agent but I’m also equipped with the experience and judgment to be the next commander in chief.”

On the campaign trail, Obama gently reminds voters that Clinton and Edwards are not so experienced: She is a second-term senator who has never run a government or business. Edwards served one term in the Senate.

“I’ve been in public office longer than Hillary Clinton has,” he said Monday, counting his seven years in the state Senate and not counting Clinton’s three decades in public life with her husband. “I’ve been in public office longer than John Edwards has.”

He has a relatively thin resume, but it’s not without accomplishments – working across party lines to change ethics, death penalty and racial profiling laws in Illinois. Ethics and nuclear proliferation are his signature issues in the Senate.

“I’ve got a track record, not only in the state legislature but in Washington for taking on tough issues and getting something done,” he said.

“I want to make sure that during the course of these next four or five months we talk about experience and judgment, not just in the ways that Washington has defined it but in the ways the people outside Washington understand it,” Obama said.

That assumes he can wrestle control of the campaign narrative from Clinton and his other battle-tested rivals – quite a stretch.

“If we’re able to do that,” he said, “then we will win.”

If he can’t, he won’t.