Commentary: Sam Says Palin's Performance Will Be a Referendum on McCain's Judgment

The following is a commentary by ABC News' Sam Donaldson. Click here to view a video version of his latest essay.

The presidential election could be on the line tonight with John McCain's chance to win in November dependent on how his running mate does in Thursday evening's vice presidential debate.

Yes, you ask, how prepared is Gov. Sarah Palin to be president of the United States if fate should require her to assume the office? She has stumbled badly in the few interviews she has given. And going into tonight, the news for her is not good.

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In our ABC News/Washington Post poll out today, 60 percent of the public now thinks she is not prepared to be president, whereas 47 percent thought she was ready early last month.

So, you say, tonight's debate is all about Palin. No, that's not it. Tonight's debate is all about McCain and the question the debate may help settle is not so much about her judgment as about his.

We used to sit around and discuss why it was that in an overwhelmingly Democratic year by all the public indexes, the Republican nominee was running neck and neck with his Democratic rival. We said it had to be to some extent because of Barack Obama's weaknesses, but mainly because of John McCain's perceived strengths. But as Palin has stumbled and bumbled her way along the campaign-media trail, the strong argument for McCain -- his mature judgment forged in war and peace that gave people confidence that he was prepared to make the tough decisions -- has come more and more into question, not helped, of course, by his "bull in the china shop" tactics during the recent financial crisis.

All Sen. Joe Biden has to do tonight is show up and avoid some terrible gaffe. But Sarah Palin has to perform in a way that will cause voters to say, "You know something, she got off to a terrible start but it looks like she's finding her sea legs and I'm going to take a second look. Maybe John McCain was right to pick her after all."

If she performs badly, however, the McCain-Palin ticket could lose the election not because of what that says about her but because of what it says about him.

Sam Donaldson, a 41-year ABC News veteran, served two appointments as chief White House correspondent for ABC News, from January 1998 to August 1999 and from 1977-1989, covering Presidents Carter, Reagan and Clinton. Donaldson also co-anchored, with Diane Sawyer, "PrimeTime Live," from August 1989 until it merged with "20/20" in 1999. He co-anchored the ABC News Sunday morning broadcast, "This Week With Sam Donaldson & Cokie Roberts," from December 1996 to September 2002. Currently, Donaldson appears on ABC News Now, the ABC News digital network, in a daily show, "Politics Live."