By any relevant historical measure, the 2012 presidential election should be something close to a layup for Republicans, a piece of cake, no heavy lifting.

The incumbent Democrat in the White House is presiding over the longest economic slump in more than 70 years, inherited to be sure but his responsibility now after more than three years in office. Unemployment remains stubbornly above 8 percent. Public optimism about the direction of the country and the future of the economy is in the pits.

Incumbents don’t normally win re-election with that kind of baggage.

All the GOP would seem to need to exploit this advantage are: 1) a clear, credible message about how we got into this mess and how to fix it, and 2) absolute honesty on both counts.

But if the storm-tossed Republican National Convention in Tampa is any measure, the Grand Old Plutocrats are in danger of blowing it.

The Tuesday night appearance of Ann Romney and Gov. Chris Christie provided the convention’s “Entertainment Tonight”-style highlights and worked pretty well. But it was the anxiously awaited oration of Rep. Paul Ryan, the party’s veep nominee and reputed Big Brain on government economics, that was to make the substantive case for a Republican return to power.

Instead, what the Ryan speech delivered was a field day for fact-checkers among the journalists covering the conclave and an unintended bonanza for Democrats.

Sure, the GOP delegates loved it all. But they’re not the audience Ryan and Mitt Romney need; they’ve already got them, presumably. They need undecided independents and wavering Democrats. But it’s doubtful that Ryan’s rhetoric, once put through the fact-check wringer, will do much to win them over.

For starters, there’s his now widely debunked claim that President Obama oversaw the shutdown of an auto plant in his Wisconsin hometown. Trouble is, the plant was shuttered in 2008 and Obama didn’t take office until Jan. 20, 2009. How inconvenient!

Consider a couple of his other whoppers. Obama, he said, will take some $700 billion out of Medicare to help fund the Affordable Care Act, aka Obamacare. It’s dastardly, Ryan implied. But he neglected to mention that his own budget pulls almost exactly the same amount from Medicare. Just an oversight, right?

He also laid into Obama over the president’s failure to embrace the Simpson-Bowles commission proposal for dealing with the deficit. A presidential lack of leadership, Ryan charged, with some justification.

But his criticism would have had more bite if he’d acknowledged that he, too, took a walk on the Simpson-Bowles proposal. And he was a member of the commission. (Ryan liked its spending cuts, but was frightened away by tea party opposition to its tax increases. Another congressional profile in courage by another self-proclaimed reformer.)

There’s more. Some of Ryan’s most telling political points involved the burst of federal spending under Obama. Actually, Ryan knows something about big federal spending and borrowing. Almost without exception, he voted for the explosion of spending under President George W. Bush, including the now-controversial tax cuts, the Iraq and Afghanistan wars, the huge prescription-drug expansion of Medicare, even TARP, the bank bailout now considered almost treason by some in the GOP.

Ryan was just another of those big-spending Bush Republicans he now disavows. When did he change and why, a television analyst was asked. “When a Democrat was elected president,” was the response.

Will Ryan’s elaborate fabrications hurt him and the GOP? Only if enough voters paid close attention to the big Republican show and the reaction to it in the media and academia — of which, after years of covering these things, I’m doubtful.

For the moment, Ryan gets a break. Mitt Romney’s acceptance speech, not great but not bad either, has pushed Ryan’s losing battle with the truth into the background for a while. But the GOP ticket will be under a close fact-check watch from now until the votes are tallied in November.

Which should cause the Romney-Ryan team to clean up its act, right? Not necessarily, if you believe Neil Newhouse, the team’s chief pollster. As he reportedly snapped to an interviewer in the wake of Ryan’s truth-trashing, “We’re not going to let our campaign be dictated by fact-checkers.”

There. Take that!

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