The short answer is that they don’t matter much at all in terms of the election outcome. Unless one of the running mates clearly disqualifies himself as a plausible president, it should have very little effect on voters’ intentions. The only reason that voters would have to abandon one ticket or support the other is if the vice presidential nominee demonstrated such obvious unfitness for the job that it calls the presidential nominee’s judgment into question. If a VP nominee implodes during the debate, that tells the public that one of the most important decisions the presidential nominee has already made was likely wrong. It’s possible that a major error by the incumbent Vice President could be more damaging, because it will alert voters to the fact that the person already in the office may not have been as prepared as they previously thought. That said, no one expects either candidate to make any fatal mistakes tonight.

The other question that is being asked right now is whether tonight’s debate will “matter” by reviving the Obama campaign’s flagging fortunes. A competent, effective performance by Biden would provide reporters with a new story for a few days, and it could produce some positive coverage for their campaign, but there is also the possibility that a very good performance by Biden would serve to draw attention to the gap between his debate outing and Obama’s. Such comparisons would likely be unflattering to Obama. One can already imagine how Republicans might exploit this considering their very low opinion of Joe Biden.

Could the debate tonight put the Romney campaign on the defensive? Maybe, but it’s worth thinking about whether the reverse would be true if the first presidential debate had gone very differently. Suppose the first debate had gone the other way and Romney had been perceived as the loser by a wide margin. Would a masterful performance by Paul Ryan save things for their campaign, or would it instead unleash a new wave of complaints about the presidential nominee? If Ryan’s performance were superior to a poor showing by Romney, that would just revive all the old doubts about Romney’s competence. “It’s too bad Paul Ryan isn’t our presidential nominee,” the Ryan boosters would say yet again, and media coverage would feed on the intra-party bickering just as they have fed on the post-debate panic among Obama supporters.

Having said that, Obama and Biden would gain nothing from a weak showing by Biden. Both candidates will probably serve their campaigns well. Because Ryan has not been in a debate at this level before, more people will be looking for missteps from him. Whether his supporters want to believe it or not, Ryan is at a disadvantage on foreign policy, and that is where he could get into trouble. I expect that he will be disciplined and careful enough not to say anything too alarming or foolish, and he will probably keep coming back to generic affirmations of “peace through strength,” opposing cuts in military spending, and the need for “leadership.” There aren’t any voters at this point that will abandon Romney-Ryan over Ryan’s lack of foreign policy experience. Anyone concerned about that has already taken it into account. The only danger for Ryan is if he conveys that lack of experience by saying something completely absurd. Expectations for Ryan should be much lower than they are, but inexplicably many Republicans seem intent on raising them because of their overconfidence in his talents.

P.S. Rob Farley and I recently talked about foreign policy in the vice presidential debate for bloggingheads: