DEBATES GIVE OBAMA AN EDGE…. All of the snap polls and focus groups showed Barack Obama winning each of the debates against John McCain, but the lasting impact of the debates matters far more.

John McCain improved his debate scores in his final encounter with Barack Obama, but not enough to challenge Obama’s dominance across their three meetings — an advantage that’s improved Obama’s image well beyond his core supporters.

Likely voters by 3-1 say they have a better rather than worse opinion of Obama because of his debate performances. It’s helped him in key groups including white voters overall, working-class whites, independents, married women, white Catholics — even among conservatives and perhaps evangelical white Protestants, core Republican constituencies.

Overall, 36 percent of likely voters in this ABC News/Washington Post poll say they have a better opinion of Obama as a result of the debates, 12 percent worse. That compares with McCain’s result: Just 20 percent think better of him, vs. 26 percent worse.

The impact among self-described independents was especially interesting — 33% said the debates led to think better of Obama, while only 13% said the opposite. For McCain, 19% of independents said the debates made them think better of the Republican, while 28% said the opposite.

A few weeks ago, James Fallows noted the 1960, 1980, and 1992 races, and importance of the debates in each. “In each of those cases, a fresh, new candidate (although chronologically older in Reagan’s case) had been gathering momentum at a time of general dissatisfaction with the ‘four more years’ option of sticking with the incumbent party. The question was whether the challenger could stand as an equal with the more experienced, tested, and familiar figure. In each of those cases, the challenger passed the test — not necessarily by ‘winning’ the debate, either on logical points or in immediate audience or polling reactions, but by subtly reassuring doubters on the basic issue of whether he was a plausible occupant of the White House and commander in chief.”

Now that the debates are finished, I’d argue that Obama excelled on both counts — he “won” the debates and cleared the credibility test.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.