Rarely has a vice presidential debate been as crucial as the one between Vice President Biden and Rep. Paul Ryan on Thursday night will be. After Mitt Romney’s lopsided victory over President Obama in Denver last week, the exchange will arrive at a fluid and potentially pivotal moment in the campaign.

For the Obama team, Thursday will offer an opportunity to short-circuit the advances Romney has made since the first presidential debate. For the GOP, Ryan will have a chance to piggyback on Romney’s performance and solidify the gains their ticket has made in recent days.

The stakes are also higher than usual because the participants have a standing beyond their roles as running mates. They are real players, not potted plants or a sideshow to the main event.

Biden is vice president and therefore fully accountable for what has gone right and wrong on Obama’s watch. He is a central player in the administration who has not shirked from offering the president unvarnished advice. He also speaks with the credibility of someone who has been in public life for four decades, with wide experience in foreign and domestic policy.

Ryan, too, is more than just a vice presidential nominee plucked from obscurity to fill out a national ticket. The House Budget Committee chairman may have a limited public profile nationally, but he has served seven terms in Congress and, of far greater significance, his peers consider him the intellectual leader of the Republican Party for his work on the budget and the economy.

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If you doubt that, recall this: Long before Romney was the GOP nominee, Obama was attacking Ryan and his fiscal blueprint, an early foreshadowing of the 2012 campaign that has come into clearer focus this fall.

Some vice presidential debates are mere entertainment relative to the overall race, enjoyable but incidental to the fundamentals of the campaign. That was certainly the case four years ago when the forum, which drew a huge audience, was all about Sarah Palin, whose candidacy had captured the imaginations of friends and foes alike.

But the outcome of that exchange — a tie or modest victory for Palin in the eyes of many pundits and a win for Biden according to the public — did nothing to shift a race that already had moved away from Sen. John McCain (R-Ariz.) and safely toward Obama. Thursday’s debate at Centre College in Danville, Ky., could be different.

National public polls show movement toward Romney since Denver. A Pew Research Center poll released Monday highlighted a big gain for Romney. Gallup began posting a sample of likely voters from its tracking poll and Tuesday’s showed Romney leading 49 percent to 47 percent. Among registered voters, Obama was ahead 49 percent to 46 percent.

GOP enthusiasm is up, which is good news for Romney. But more evidence is needed to know how much the landscape in the battleground states has been changed by the debate. CNN released a poll of Ohio on Wednesday that showed two things: Romney has narrowed the gap there, but the president is still ahead among likely voters by 51 percent to 47 percent. Other battleground surveys are expected by week’s end, and they should give a clearer picture of the state of play.

Romney advisers say the key battlegrounds are now within the margin of error, a significant shift from two weeks ago, when Obama led in Ohio, for example, by six to 10 points, depending on the survey. They are far more bullish today than they were a few weeks ago amid Romney’s difficulties.

Obama advisers say they, too, have seen some movement toward Romney but make two points to counter what the Romney team is saying. In Ohio, for example, they say that they still believe the president holds a real lead and that they remain narrowly ahead in a number of other states.

They argue that the big margins of two weeks ago were always destined to tighten, given everything that is known about these states from past campaigns. The debates, they say, accelerated a process that was likely to happen sometime in October.

But from their research, the race has begun to settle down. Romney is not continuing to gain ground. “Romney has consolidated some of the gains he was going to get anyway,” said White House senior adviser David Plouffe. “We weren’t going to win battlegrounds by 10 points.”

The campaigns aren’t playing the expectations game ahead of this debate as vigorously as they did before the first presidential debate. Biden is knowledgeable and an experienced debater. Republicans expect he will give Ryan a tough exchange. Ryan also is considered smart, substantive and authentic. Some Democrats regard him as a better communicator than Romney. They don’t anticipate he will stumble.

Ryan has several possible weaknesses. He knows the budget to such a level of detail that he could fall into wonk-talk if he’s not careful. What works on the House floor or in an inside-the-Beltway debate won’t necessarily work with a television audience of 50 million or more. As one Republican noted, Ryan has to resist being chairman of the House Budget Committee on Thursday night.

Nerves also could be an issue. Ryan has never debated on a stage this big. He has much at stake. Beyond this election, there is the question of his future as a national Republican leader — a possible presidential candidate in 2016 if Romney loses.

Biden’s potential weaknesses are well known. One is a propensity to be windy, although he was anything but that against Palin. He has a history of verbal gaffes, but mostly when he’s unscripted or lets his guard down, which shouldn’t be the case Thursday. He could appear condescending or patronizing to his younger rival. He could be too aggressive. He also must defend Obama’s record on the economy, which the president did not do effectively last week.

Ryan said this week that he expects Biden to be forceful on Thursday to make up the ground that Obama ceded to Romney last week. Biden will attempt to force Ryan to defend what the Obama campaign considers Romney’s dishonest performance in Denver.

The vice president’s attacks probably will focus on Ryan’s budget, which Romney has embraced only in part, and he’s likely to try to make Ryan the issue as much as Romney. Ryan’s challenge will be to make an appealing case for his worldview while making clear that Romney is the head of the ticket and running on his own platform.

The next presidential debate will be held Tuesday at Hofstra University in New York, but what happens Thursday in Kentucky is likely to have a big effect on how things look by the time Obama and Romney meet again.

For previous columns by Dan Balz, go to postpolitics.com.