"There are two questions we will never have to ask ourselves, 'Who is this man?' and 'Can we trust this man with the presidency?'"
-- Fred Thompson on John McCain, Sept. 2.

WASHINGTON -- This was the most effective line of the entire Republican convention: a ringing affirmation of John McCain's authenticity and a not-so-subtle indictment of Barack Obama's insubstantiality. What's left of this line of argument, however, after John McCain picks Sarah Palin for vice president?

Palin is an admirable and formidable woman. She has energized the Republican base and single-handedly unified the Republican convention behind McCain. She performed spectacularly in her acceptance speech. Nonetheless, the choice of Palin remains deeply problematic.

It's clear that McCain picked her because he had decided that he needed a game-changer. But why? He'd closed the gap in the polls with Obama. True, that had more to do with Obama sagging than McCain gaining. But what's the difference? You win either way.

Obama was sagging because of missteps that reflected the fundamental weakness of his candidacy. Which suggested McCain's strategy: Make this a referendum on Obama, surely the least experienced, least qualified, least prepared presidential nominee in living memory.

Palin fatally undermines this entire line of attack. This is through no fault of her own. It is simply a function of her rookie status. The vice president's only constitutional duty of any significance is to become president at a moment's notice. Palin is not ready. Nor is Obama. But with Palin, the case against Obama evaporates.

So why did McCain do it? He figured it's a Democratic year. The Republican brand is deeply tarnished. The opposition is running on "change" in a change election. So McCain gambled that he could steal the change issue for himself -- a crazy brave, characteristically reckless, inconceivably difficult maneuver -- by picking an authentically independent, tough-minded reformer. With Palin, he doubles down on change.

The problem is the inherent oddity of the incumbent party running on change. Here were Republicans -- the party that controlled the White House for eight years and both houses of Congress for five -- wildly cheering the promise to take on Washington. I don't mean to be impolite, but who's controlled Washington this decade?

Moreover, McCain was giving up his home turf of readiness to challenge Obama on his home turf of change. Can that possibly be pulled off? The calculation was to choose demographics over thematics. Palin's selection negates the theme of readiness. But she does bring important constituencies. She has the unique potential of energizing the base while at the same time appealing to independents.

This is unusual. Normally the wing-nut candidate alienates the center. Palin promises a twofer because of her potential appeal to the swing-state Reagan Democrats that Hillary Clinton carried in the primaries. Not for reasons of gender -- Clinton didn't carry those because she was a woman -- but because more culturally conservative working-class whites might find affinity with Palin's small-town, middle/frontier American narrative and values.

The gamble is enormous. In a stroke, McCain gratuitously forfeited his most powerful argument against Obama. And this was even before Palin's inevitable liabilities began to pile up -- inevitable because any previously unvetted neophyte has "issues." The kid. The state trooper investigation. And worst, the paucity of any Palin record or expressed conviction on the major issues of our time.

McCain has one hope. It is suggested by the strength of Palin's performance Wednesday night. In a year of compounding ironies, the McCain candidacy could be saved, and the Palin choice vindicated, by one thing: Palin does an Obama.

Obama showed that star power can trump the gravest of biographical liabilities. The sheer elegance, intelligence and power of his public presence have muted the uneasy feeling about his unreadiness. Palin does not reach Obama's mesmeric level. Her appeal is far more earthy, workmanlike and direct. Yet she managed to banish a week's worth of unfriendly media scrutiny and self-inflicted personal liabilities with a single triumphant speech.

Now, Obama had 19 months to make his magic obscure his thinness. Palin has nine weeks. Nevertheless, if she too can neutralize unreadiness with star power, then the demographic advantages she brings McCain -- appeal to the base and to Reagan Democrats -- coupled with her contribution to the reform theme, might just pay off. The question is: Can she do the magic -- unteleprompted extemporaneous magic, from now on -- for the next nine weeks?