The Democratic National Convention, where Barack Obama badly needs to accomplish four tasks:

Define himself. Obama fans may have a tough time believing this, but millions of Americans still view the guy as a stranger. And they're not necessarily wild about entrusting the White House to a stranger — especially one who has been defined by Republicans as a silver-tongued celebrity with insufficient government seasoning. And that's just the polite version.

Obama needs to use his four-day infomercial to wrest control of his own personal story, to persuade the skeptics that, as an all-American success story, he has the requisite heart and wisdom to lead. Many people still don't know that he was raised by a single mother on food stamps, or that his white Kansas grandfather fought in World War II, just as many people, back in 1992, didn't know that rookie candidate Bill Clinton had endured a difficult home life in hardscrabble Arkansas.

Clinton used his first convention to reframe his biography, with strong assistance from an artfully produced movie about himself. Obama needs to do the same — he is indeed readying a movie about himself — and thereby show the audience at home that he understands the fundamental American journey. In short, he needs to forge an emotional connection with the millions of skeptics who typically see him as an aloof Ivy League egghead.

Address the experience issue. He's stuck with a thin resume — he didn't get to Washington until 2005 — which means he'll have to dispel the doubts some other way. But it won't be enough for him to utter fine phrases about his own judgment in the acceptance speech on Thursday night; we all know he speaks well in front of stadium crowds, so in a way his rhetorical success will be discounted.

Rather, he needs to signal to viewers — particularly in his choice of running mate — that as president he would be buttressed by a seasoned team of policy players, with national security as priority one. He needs to ensure that various Democratic wise-persons (Richard Holbrooke, Madeleine Albright, Sam Nunn, Robert Rubin) are singing his praises in interviews.

And speaking of key Democratic figures: Light a fire under the Clintons. It's no surprise that Bill and Hillary have done virtually squat to boost Obama's candidacy. Narcissists (Bill in particular) have a notoriously tough time yielding the spotlight. But playtime is over. For Obama's sake, Bill needs to park his snit and make a persuasive case at the convention for why he believes (or at least is willing to say he believes) that Obama is ready to be president.

Obama has already pandered to them enough, giving Bill the mike on one night and Hillary the mike on another night, so it would greatly help the Democratic ticket if they deigned to educate Hillary's bitter-enders on the facts of life: namely, that sitting on their duffs in November would enhance the election prospects of John McCain, and thus imperil the very issues (universal healthcare, abortion rights, and many more) they profess to hold dear.

Hillary, mindful of her own career, would prefer not to be blamed in November if the Democrats lose this race in a cliffhanger.

In other words, Obama needs a convincing show of unity, precisely the opposite of the worst Democratic embarrassment, back in 1980, when President Jimmy Carter literally followed Ted Kennedy around the stage in the futile hope of getting his vanquished rival to clasp hands. Obama needs the Clintons on board now, even if they wind up plaguing him for years.

Define John McCain. By all the traditional political metrics, this election is supposed to be a referendum on the incumbent party that has mismanaged America at home and abroad — yet, late this summer, the incumbent party has somehow succeeded in framing this race as a referendum on Obama.

At his convention, Obama needs to reverse the plot arc. He needs to borrow a page from the longtime Republican playbook and tear the bark off McCain. And strip him bare for the next 70 days.

Check out any poll. Most Americans are ticked off about the state of the union. Obama needs to use his convention to harness this anger. Democrats tend to think that having the issues on their side is enough; Republicans demonstrate that simple, visceral, repetitive messaging trumps the issues almost every time. Therefore, Obama would be remiss if his convention failed to employ this shorthand at every opportunity:

McCain is the same as a third term for Bush.

Out-of-touch McCain is so rich, he doesn't know how many houses he has.

McCain and Bush have made America weaker.

If sustained for the next two months, these messages could put Obama back on offense, which is where winners live.

At times, Obama has seemed to assume he can impose a bipartisan "new politics" merely by dint of his presence. The problem is, the people who want to take him down are playing the old politics, and that game still works. If Obama fails this week to demonstrate a passion for combat, to dispel the widely held suspicion he is a mere babe in the woods surrounded by predators, he may well be toast in November.

Dick Polman is a columnist for the Philadelphia Inquirer. Reach him at dpolman@phillynews.com.