When Sen. Joe Biden and Alaska Gov. Sarah Palin face off in Thursday night’s vice presidential debate, each will be competing against a unique set of challenges, expectations and pitfalls.

Republicans say Biden, debating in favor of Democratic candidate Barack Obama’s record, will be contradicting policy disagreements he made during the nomination battle. They will be watching for flip-flops and for the Delaware senator’s tendency to exaggerate or put his foot in his mouth.

In the days leading up to the debate in St. Louis, both campaigns have sharpened their knives when it comes to the opposing running mate, and a number of outside groups have taken aim at the No. 2s. But voters tuning in will likely be holding the two candidates to two very different standards.

Biden faces almost the exact opposite expectations Palin does. The senator has been around Washington for 30 years, Republicans opponents say, and he should have no problem handing in a strong debate performance. Any failure to do so will be because he has fundamental disagreements with Obama or because he exaggerates the Democratic ticket’s policy positions.

“The minute he comes out and starts talking about how raising taxes is patriotic, then people are going to be turned off immediately,” one aide to John McCain said.

The McCain-Palin official argues Biden has proven that he is still vulnerable to gaffes and misstatements that can harm Obama, particularly if he offers his own policies that differ from the Illinois senator’s.

“The biggest problem that Biden faces is that he is now supporting a record that he adamantly opposed six, seven, eight months ago,” the aide said. “He is going to have to stick up for a record that he doesn’t believe in.”

The aide, however, brushed aside Democratic concerns that Biden, known by reporters to be garrulous and informal to a fault, might come off as condescending or even bullying as he addresses the relatively unknown governor, who was able to win a significant wave of sympathy from conservatives who viewed her treatment by the media to be unfair.

“We don’t think that Biden is going to do us any favors,” the aide said. “I mean, the guy has been debating for 30 years.”

Republican strategist Todd Harris, who worked for McCain in 2000, said Biden’s personality, however, does carry risks.

“Biden needs to be very careful that he doesn’t look condescending, that he doesn’t look arrogant and that he doesn’t speak in Senate-ese,” Harris said.

But the Obama camp expressed confidence in its No. 2.

Nick Shapiro, an Obama spokesman, said Biden “deeply shares” the Illinois senator’s commitment to the middle class.

“Unlike John McCain, who has fought for the deregulation and corporate interests that got us into this mess, Joe Biden has fought for the little guy and ensured everyone played by the rules. He’s also fought for a responsible end to the Iraq war so we can bring our troops home and focus on real terrorist threats,” Shapiro said. “Unless Sarah Palin offers a distinctly different vision from John McCain on the economy or national security, she’s just more of the same. So far, she’s failed to do anything but offer blind support for the same failed policies of the past.”

Democratic strategist Mark Kornblau, who worked for former Sen. John Edwards (D-N.C.) both during his 2008 presidential run and his 2004 campaign as Democratic vice presidential nominee, said Biden’s focus should not be on Palin but on the top of the ticket.

“Sen. Biden needs to be a prosecutor,” Kornblau said. “He should ignore Palin and make an aggressive, cohesive case against John McCain — indicting him on charges of repeatedly exercising bad judgment and pursuing a third Bush term.”