If Palin gives a crackerjack speech and impresses mainstream, centrist voters as well as the conservative, devoutly Christian base of the Republican Party that already loves her, she may well play a key role in propelling Republican standard-bearer Sen. John McCain of Arizona into the White House.

But if Palin fails to rise to the challenge and comes across as superficial, fearful, inexperienced or lacking in intellect, she could crystallize the potentially decisive 6-point lead that Democratic presidential nominee Sen. Barack Obama of Illinois has opened up in national opinion polls.

Palin's initial glow has been tarnished first by the ferocious personal attacks and snide electronic gossip about her and her family, and even by the news that her 17-year-old daughter Bristol is pregnant and is going to keep the baby instead of opting for an abortion.

But Palin has already served an invaluable key role for McCain. The ferocious attacks on her alleged "lack of experience" running Alaska for the past two years have allowed McCain and his campaign strategists to counter by putting Obama's lack of experience on the table and contrasting it with Palin's highly successful record as chief executive of a U.S. state.

It is a bold ploy, and it could prove surprisingly effective. After all, although McCain is one of America's most acclaimed war heroes, he has been in the Senate for more than two decades and has no executive experience himself. Neither does Obama or his running mate, 30-year Senate fixture Joe Biden of Delaware.

Obama has proudly and repeatedly cited his four months of experience as a community organizer in Chicago more than a decade ago, but Palin has actually run a state of the union several times the size of Texas, a state that provides 20 percent of the oil and natural gas produced in the entire country. She oversees a state-wide government of 24,000 employees, 10 state-wide agencies and an annual budget of $10 billion. The Republican campaign announced Wednesday it was bringing out a new advertisement highlighting these points.

The Republican campaign is also betting highly on the no-holds-barred support of independent Sen. Joe Lieberman of Connecticut, who was Democratic presidential nominee Al Gore's running mate in 2000.

Lieberman is the opposite kind of figure from Palin in every way. He, like Biden, has been a fixture on the Democratic side of the Senate for decades.

Lieberman is high-profile traditional Jewish, whereas Palin is passionately Christian, and he has always been a pro-choice liberal on abortion, whereas Palin rejects it in the name of "the culture of life."

Lieberman in fact is likely to have little national appeal or impact. But he could convince small but still useful numbers of centrist voters to defect to McCain. And most of all, his support will nail down further crucial Jewish support for McCain in Florida, where elderly, more politically centrist or conservative Jewish voters tend to settle. But don't look for Lieberman to give McCain any additional significant inroads in New York, California or Illinois. The Jewish communities in all three of those states are overwhelmingly liberal, pro-choice and rock solid for the Democratic Party.

President George W. Bush suffered a further indignity Tuesday night. His keynote address to the Republican National Convention in Minneapolis was swept off the board when the first night was eliminated entirely owing to concerns that Hurricane Gustav might prove a serious threat to the Gulf Coast.

That didn't happen, but when the convention finally got under way Tuesday, Bush was limited to an eight-minute video address. The president, who has always been a shrewd domestic political operator, may well have ruefully acknowledged that given his current poll approval ratings, the less national exposure that was associated with him at the convention, the better.

However, it says much for Bush's fading clout and perceived importance two months away from the presidential election that Palin -- a woman perhaps one American in 10,000 had even heard of outside Alaska before Friday -- now exceeds him in importance for the Republican presidential campaign.

The McCain strategy team appears uneasy about Palin now after the initial euphoria for her. On the one hand, they are fiercely counterattacking against the Democrats with their new television advertisement contrasting her to Obama. On the other hand, they have kept her off CNN's "Larry King Live," and King has made his career playing cozy softball with the movers and shakers. Clearly, the Republican campaign's immediate focus is to deliver a positive defining image of Palin with Wednesday night's convention speech.

She will have her work cut out for her, as Obama finally got the convention bounce that had eluded him for most of last week: He had a lead of 6.4 percentage points in Real Clear Politics' average of major polls Wednesday. Does that give him the momentum he needs heading into the true campaign? Or can Palin and McCain eliminate it or cut it back?

Right now, Palin carries more political weight on her shoulders than President Bush or John McCain in shaping the conditions under which McCain enters the final two-month stretch of his long race with Obama.