Who woulda thunk that Joe Biden, of all people, would drag a onetime rockstar president over the finish line? Only a few months ago, plenty of Democrats wanted the vice president dumped from the ticket in favor of Secretary of State Hillary Clinton. Now he is saving Barack Obama’s bacon.

Historically, vice-presidential debates have had little impact on the outcome of presidential elections, but there are exceptions and 2012 may prove to be one of them.

In 2004, the Democratic challenger, John Kerry, was up 8 points in several polls after besting President George W. Bush in their first debate. But then Vice President Dick Cheney drove home Bush’s national-security message in his debate with John Edwards and won on points, not likeability, which was never Cheney’s strong suit. Bush’s polling stabilized and he went on to win a close election.

Unless Obama lays eggs in the two next debates, I suspect the same dynamic will be at work this year. Biden may have irritated some voters — the instant polls were split — and Paul Ryan’s smooth and intelligent performance makes him a likely Republican nominee for president in the future. But the big takeaway from this contest will be that Biden stopped, or at least slowed, Mitt Romney’s momentum, re-energized panicky Democrats and scored heavily with two key constituencies: senior citizens and women.

The contrast between Biden’s performance and that of his boss in Denver a week earlier couldn’t have been starker. Where Obama never missed an opportunity to miss an opportunity (as former Israeli Foreign minister Abba Eban liked to say about the Arabs), Biden jumped on every one of Ryan’s arguments.

When Obama’s debate-prep team analyzes which of Biden’s counterpunches landed best with focus groups, the president will cherry-pick the answers and try to deliver a calmer, more presidential and more selective version of his running mate’s performance.

This isn’t to claim that Biden “won”; unlike the presidential forum in Denver, the verdict on the Kentucky contest broke down along party lines, though the conservative pundit Dick Morris wrote in a mid-debate Twitter post: “can’t believe how weak Ryan is.”

Morris knows that Romney must hold senior citizens, the only age group that went for John McCain in 2008. Unfortunately for the Republicans, Biden clarified for seniors (most of whom turn out to vote) that Democrats are the protectors of Social Security and Medicare while Republicans consistently favor privatizing and voucherizing those hugely popular programs.

When Biden spoke about Medicare at the debate, he turned straight to the camera and reminded seniors that Obama’s health care law had brought them $600 a year in reduced prescription-drug costs (by closing the so-called donut hole, a term that may be unclear to most voters, but not to the elderly) and by providing coverage for preventive-care visits.

The Republicans will call that pandering, but it no doubt sounded good in Florida. There, Ryan’s history of championing George W. Bush’s privatization plan in 2005 is scary for seniors who, then as now, don’t want their retirement savings exposed to the fluctuations of the stock market. Biden’s “c’mon, guys” common-sense appeal probably scored with voters wondering whom to trust on this issue.

The same dynamic was at play on abortion, which wasn’t mentioned in the presidential matchup in Denver and is of critical importance to undecided women voters who had been moving toward Romney in the last week.

Republicans may say this is a tired evergreen for Democrats and that Presidents Ronald Reagan, George H.W. Bush and George W. Bush won with anti-abortion views. But when the health of Justice Ruth Bader Ginsburg, a 79-year-old survivor of pancreatic cancer, is all that stands between women and the loss of their reproductive freedom, that issue has returned to the forefront of the campaign, partly thanks to Biden.

It is likely that Romney, whose views on abortion have been all over the place, will have to say clearly in his second debate with Obama whether his presidency would mean a reversal of Roe v. Wade.

Biden’s other achievement was to expose some of the hypocrisy at the heart of the Republican argument. He nailed Ryan for seeking stimulus money for his congressional district while railing against the program, and for assailing a lack of security at U.S. embassies after he voted to reduce the relevant funding by $300 million.

Romney will continue to gallop toward the center, where presidential elections are won. It will be up to Obama to use some of Sheriff Joe’s ammo to cut him off at the pass.

(Jonathan Alter is a Bloomberg View columnist and the author of “The Promise: President Obama, Year One.” The opinions expressed are his own.)

Photo credit: AP/Pablo Martinez Monsivais