Tonight’s TV debate between Vice President Joe Biden and his Republican challenger Paul Ryan will be exciting and important, although it will not affect the election outcome in any significant way.

That’s the belief, at least, of Gallup, whose recent report shows that none of the last eight debates between vice presidential candidates, between 1976 and 2008, had a major impact on the election results. In the 2008 election, for example, when Joe Biden debate Sarah Palin, support for the Republicans fell by only 1 percent after the debate, while support for the Democrats rose by 2 percent.

The race has tightened since Romney’s strong debate last Wednesday coupled with Barack Obama’s close to catastrophic performance. But, warns Nate Silver on his excellent political statistics blog FiveThirtyEight, in spite of Romney’s great success in the polls after the debate, he has not taken the lead in one of the ten “swing states” or “battle ground states”.

Romney, according to Silver, may have improved his chances of winning the election by 15 to 20 percent a result of his victory in the first TV debate, but:

”The more troubling sign for Mr. Romney, however, is that although he’s made gains, he does not seem to have taken the lead in very many state polls. That trend, if anything, has become more entrenched. Of the half-dozen or so polls of battleground states published on Wednesday, none showed Mr. Romney ahead; the best result he managed was a 48-48 tie in a Rasmussen Reports poll of New Hampshire.”

So Biden-Ryan debate tonight becomes very important for the remaining four weeks of the election campaign. For the Democrats the goal is to regain the lead and the momentum before Obama’s failed debate and for the Republicans it’s about not losing their new momentum. How will it happen?

I like Matt Miller’s advice to Biden in his weekly column in the Washington Post about what the vice president must do to win the debate. It will not be enough to accuse Romney of being a “liar” – more is needed.