Approaching the site was like driving to an Obama rally: you passed a line of people, three or four abreast, going on seemingly forever through parking lots, sidewalks, and neighborhoods.  The Fairfax event turned out to be McCain’s biggest rally ever, with an estimated 23,000 people.

Fired-up people, too.  “A month ago, I wasn’t very enthusiastic about the race, but now I am,” a woman from Arlington told me.  “I was a rather disenchanted Republican, as were many of my friends.”

As we talked, it became clear that “disenchanted” was her polite way of saying she couldn’t stand John McCain.  “I’m a Reagan conservative and he’s not,” she said succinctly.  Now, with Palin on the ticket, she’s on board.

The scene in Fairfax was replayed at other joint McCain-Palin rallies around the country, drawing positive coverage in local papers and on TV.  Campaign officials were ecstatic.  But their enthusiasm couldn’t hide the fact that the other side of Palin’s debut — her introduction to the national press — has been a mess.

Palin’s performance in her first TV interview, with ABC’s Charles Gibson, wasn’t very good, but she got a pass in many quarters, in part because viewers could see that Gibson was literally — and figuratively — looking down his nose at her.  Palin came off as a sympathetic figure.

But there was a red-flag warning in the interview.  When Palin was asked a question about discretionary versus entitlement spending, she appeared to whiff, saying she was sure “efficiencies” could be found in Social Security, Medicare, and Medicaid.

Now, virtually every governor in America has some sort of angry story to tell about the burdens imposed on states by Medicare or Medicaid obligations.  Palin could have answered the question as the governor of Alaska, stressing her state’s situation.  Doing so would not only have made for an actual substantive answer, it would have highlighted her experience.  Instead, she had nothing to say.

Now, we’ve seen Palin’s interview with CBS’s Katie Couric.  It didn’t go well either, and not just because Palin didn’t seem to have many facts at her fingertips.

The bigger problem was that Palin could not, or would not, engage in a genuine back-and-forth with her questioner — not an insignificant concern, given the debate coming up.

For example, Couric pressed Palin to name an example of McCain fighting for more regulation of business, beyond his calls to reform Fannie Mae and Freddie Mac.  Palin, without an answer to give, generalized blandly. “He’s also known as the maverick though, taking shots from his own party, and certainly taking shots from the other party,” she told Couric.

A candidate with a better grounding might have argued that the failures of Fannie and Freddie are so fundamental to the present crisis that McCain’s proposed reforms could have averted disaster.

Or Palin might have pointed out that McCain has been at times notoriously pro-regulation — anyone heard of campaign finance?  Or Palin might have challenged Couric’s premise: Yes, lenders made a lot of bad bets, but hasn’t deregulation made our lives easier in many ways?  If you travel to another city and withdraw money from an ATM, aren’t you’re benefiting from the abolition of old regulations?

Instead, nothing.  “I’ll try to find you some and I’ll bring them to you,” Palin told Couric.

Thursday’s debate will be Palin’s chance to do better.  She’ll have a vastly larger audience than the CBS or ABC newscasts, and she’ll be appearing live and unedited.

She’ll have the chance to display the appeal she has shown on the campaign trail, but she’ll have to — have to — show more substance.

And she could use a little help from her opponent.

A couple of weeks after that big rally in Fairfax, I went to Woodbridge, Virginia to watch Joe Biden speak at Veterans Memorial Park Community Center.  With about 200 people — roughly 1/100 the size of the McCain rally — the gathering was pretty small stuff.  But what was most striking was Biden himself.

Even though the room was small and the crowd friendly, the campaign had set up a podium and teleprompter as if Biden were giving a major address.  Biden stuck carefully to his script, and when he finished, he didn’t take any questions from the crowd.  Meanwhile, aides kept the press far away.

Why the tight controls?  Because Biden just can’t seem to control what comes out of his mouth.  A few days earlier, for example, at a union event in Castlewood, Virginia, Biden said, “I guarantee you Barack Obama ain’t taking my shotguns, so don’t buy that malarkey…If he tries to fool with my Beretta, he’s got a problem.”

It was a gaffe so amazing that Jon Stewart, in what may or may not have been a joke, warned Biden that “you appear to possibly be threatening gun violence against your own running mate.”

That, plus a number of other gaffes, led to the teleprompter in Woodbridge.  But Joe Biden is Joe Biden.  The man who has been talking virtually nonstop for the last 36 years isn’t going to stop anytime soon.  Who knows what he’ll say in the debate?

Still, Palin — and Republicans — cannot count on Biden making one of his trademark bloopers in St. Louis.  Fair or not, the debate will be largely about Sarah Palin, and this is one test she must pass.

Examiner contributor Byron York is White House correspondent for National Review.