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Barack Obama's challenges at the four-day Democratic national convention in Denver fall into two categories. The first are all the normal tests set out for any non-incumbent seeking the presidency-introducing himself, defining an agenda, making a positive impression, etc. Relatively speaking, these are lay-ups.

The second are the tests unique to being a relatively inexperienced senator — and the first major party African American nominee — who won a fractious race against his party's dynastic forces and has recently dealt with the first significant poll-sliding in a career that's been defined so far by forward momentum. This is the tough stuff.

Tough stuff first.

Obama comes into the convention with the contest essentially tied after a recent McCain surge. Given that 2008 is a year in which polls suggest that the Democratic nominee should have the wind at his back, Obama must find a way to reassure the worriers in his party that he's got what it takes to win. One way to do that is to elegantly balance the convention rhetoric between a shower of love for Obama and a rhetorical beatdown of John McCain. Many Democrats believe that the party's 2004 convention spent far too much time defining John Kerry and too little taking whacks at the incumbent he was trying to beat. A well-executed four-day extravaganza that sends Obama back on to the trail with a healthy, durable and measurable bounce — and McCain into the Republican convention with a few bruises — would certainly help.

But in order to maximize his post-convention momentum, Obama has got to make sure that the manifestly bad feelings between his campaign and the Clinton team never rear their head in Denver. If any ill will enters the convention hall, there's a real danger that the rift could become the media's story of choice, and Obama can't afford to lose control of the narrative — let alone risk the support of Senator Clinton's very loyal followers.

It's clear from last week's NBC News-Wall Street Journal poll that Obama still faces a good deal of skepticism from those Clinton backers; Denver is the place where fences will be mended and votes secured — or where disunity on the convention floor could become a threat to his candidacy.

The easy stuff is still relatively hard, but at least it's entirely under the Obama campaign's control. Any non-incumbent's most important convention goal is to introduce the candidate to the country. (To paraphrase what was said about both Al Gore and Hillary Clinton when they launched their presidential campaigns, Obama is famous without being well known.) Under a spotlight more intense than anything he has experienced in his remarkable four-year rise from the Illinois state senate to White House contender, Obama must contextualize himself for the many voters who are just tuning in — to tell them about where he comes from, what experiences have shaped his values and what he has accomplished. Just as important as saying it himself is how Obama is portrayed by those who love and respect him. Michelle Obama, who has never given a speech of such magnitude despite months on the campaign trail, will be key in this respect.

It's also time for Obama and the other convention speakers to give voters a more detailed sense of what he would do as president. These presentations need to be both detailed and poetic in order to answer the basic questions that even those paying close attention don't know. What are Obama's priorities? How would an Obama-led America differ from the George W. Bush years, or from a McCain presidency? Are his proposals sensible? Affordable? Voters need to know now.

The single biggest challenge most nominees face — giving an electrifying acceptance speech — is not one of Obama's worries. He has had ample time to write his remarks, and there is not a soul on his staff, among his friends or in the press corps who doubts that the convention's final act — Obama's address to 70,000 people in a football stadium — will be anything but a smash. That should leave plenty of time for Team Obama to worry about everything else.

• Mark Halperin / Denver