- King: Americans pick presidents on Election Day, not vice presidents
- That doesn't mean the Biden, Ryan debate doesn't matter, he says
- Both sides hope for a win to give their campaign momentum, King says
(CNN) -- Yes, the debate between Joe Biden and Paul Ryan is sure to be entertaining, and it comes at an important moment in a highly competitive presidential campaign. But let's not get carried away.
Some healthy context, courtesy of Republican pollster Whit Ayres: "Dan Quayle got his clock cleaned in his VP debate and still ended up as vice president."
That's because Americans pick presidents on Election Day, not vice presidents.
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"The key for both VP candidates is to come across as competent potential presidents," said Ayres. "If they pass that test, then they have done their jobs."
Still, given the moment, even Ayres concedes Thursday night's showdown does matter some.
"A great performance by Ryan will keep Republican momentum going, and will help his post-2012 political career," he said. "But it's not likely to have a large impact on the presidential race one way or another."
That view -- the belief any impact will be limited -- is in part based on the history of debates, and in part on the timing of this one: President Obama and Mitt Romney stage their second debate just a few days after their running mates get their one debate spotlight.
So, the men atop the ticket will yield the stage only briefly.
Still, both sides have clear goals for the Biden-Ryan face-off.
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"Our sense is it matters a lot and that it is an opportunity," said a top Romney adviser who spoke to CNN only on condition of anonymity because the campaign's public line is to play down expectations.
This Romney adviser played up the generational contrast, suggesting voters in Ryan's demographic -- 30 years old to 50 years old -- voice "the brunt of the economic angst" over the job market, housing prices and America's competitive standing in the global economy.
"Ryan knows this story really well," the Romney adviser said. "He is our generational argument up against Biden's experience and age. Ryan should play well in this key demographic and he knows how to have this conversation."
Democrats, though, see an opportunity to make political gains by highlighting controversial aspects of the budget plan Ryan wrote for House Republicans.
"There are still a lot of questions about how Romney would integrate Ryan's budget plan," said Democratic pollster Margie Omero. "There is no greater way to illustrate the difference between the two camps on fighting for the middle class than with a debate between Ryan and Biden."
Share your questions for the candidates
Omero's big question: "How will Ryan talk about his proposed cuts to the safety net?" She also wonders what will happen "if Ryan gets testy" defending his plan, voicing Democratic hopes that Biden can put his GOP opponent back on his heels.
Democratic hopes for an aggressive Biden performance are amplified because of the at times lackluster showing Obama turned in during his first showdown with Romney.
Even Obama allies concede the president was hardly at his best, and that he left several potential openings on the table.
If nothing else, what Democrats want most from Biden is a morale boost.
Republican morale is on an upswing at the moment and they hope for a strong Ryan showing to keep their spirits high heading into the far more important Obama-Romney debates.
A top Romney campaign aide framed it this way when asked how much the VP debate mattered: "A little," this aide said. "It can continue our momentum and the Obama slide, or it can settle the waters."
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