Dubuque: Newt Gingrich was riding high there, for a week or so. His poll numbers were great nationally, and in battleground states such as New Hampshire and Florida, he elbowed more credible contenders—and also Mitt Romney—aside.

There really was a week there when Gingrich was the front-runner for the Republican presidential nomination.

But that’s all over now.

In the 2012 Republican race, everyone gets to be the front-runner for a week, and Gingrich has had his week.

It’s done.

Now, Gingrich is tumbling. Fast. The attacks ads paid for by Super PACS associated with Mitt Romney and Rick Santorum have surely played a part in the former speaker’s steep slide in the polls—he’s now running third, behind Ron Paul and Romney, in the Real Clear Politics survey of surveys from the past week. And is several polls he has fallen to low single digits, just above the man who might just finish ahead of Gingrich on January 3: Rick Santorum.

This is what happens when ideologues and partisans get serious about politics.

Despite the support and sympathy Gingrich has gotten from folks like Rush Limbaugh and Sean Hannity, his record was always going to disqualify him with grassroots conservatives and Republican stalwarts who want to win elections.

Gingrich plans to launch a forty-four-city bus tour of Iowa in order to grab as much free media and grassroots face time as he can for his under-financed campaign. But that will not renew his prospects.

When 2012 dawns, with the January 3 Iowa caucuses, he will be last year’s man.

Or, to be more precise, last decade’s man.

Here, then, are the top five reasons why Newt Gingrich will not be anything more than a footnote to the 2012 presidential race:

1. GINGRICH REACHED HIS SELL-BY DATE IN 1996: Born during Franklin Delano Roosevelt’s third term, Gingrich would if elected next year assume the presidency on the cusp of his 70th birthday. And unlike the conservative movement’s favorite septuagenarian president, Ronald Reagan, Gingrich has been a political player for his entire adult life. Barack Obama was two years old when Gingrich went to work on his first national campaign.

There are natural trajectories for politicians. Gingrich’s had him running for president in 1996, as the dynamic conservative challenger to President Bill Clinton. That would have been a great race between a pair of similar Southerners—smart, ambitious rascals with plenty of skeletons in their closets but also with real differences regarding the direction of the nation—but Gingrich deferred to the party bosses (and their corporate overseers) who preferred the predictability of Bob Dole.

Gingrich blinked. He missed his chance.

The same thing happened to Mario Cuomo, who should have run in 1992.

But at least Cuomo didn’t try to run in 2008.

2. GINGRICH IS A QUITTER: Stop making fun of Sarah Palin. Sure, she quit in the middle of her term as governor of Alaska, which was kind of pathetic. But Gingrich quit as Speaker of the House on the eve of the impeachment of Bill Clinton. Talk about “seduced and abandoned.” He set his fellow Republicans up for a fool’s mission, then he exited stage right.

Why did Gingrich quit not just the Speakership he had connived for a decade to obtain but his House seat? A looming scandal involving his own infidelity? Check. An inability to explain away the strategic missteps that led to the dismal finish of House Republicans in the 1998 election cycle? Check. But the real reason was that his fellow Republicans had lost faith with him as a leader.

That was a smart choice, rooted in actual experience and sincere concern about trusting the future of their party to a Gingrich. Why would Republicans abandon it now?

3. GINGRICH HAS GOT HISTORY AS A “ROCKEFELLER REPUBLICAN”: In a party that checks conservative credentials more seriously than they would have border guards check immigration papers, Gingrich committed the ultimate sin. In 1968, Gingrich was a young Republican operative looking to get his start in presidential politics. He could have signed on with the “Draft Ronald Reagan” campaign of that year. That’s what a visionary conservative would have done. He could have worked for Richard Nixon. That’s what a cautious Republican careerist would have done. But no! Gingrich served as the Southern regional coordinator for the campaign of New York Governor Nelson Rockefeller, the most liberal Republican in the field—a big-government man who backed abortion rights, opposed the Vietnam War and provided right-wingers with their preferred term of derision (“Rockefeller Republican”) for anyone who deviated from the ideologically pure path.

4. GINGRICH KEEPS GOING GREEN ON US: When he first ran for Congress in 1974, and when he ran again in 1976, at a point when the Republican right on the march (taking over the Republican platform-writing process and taking Reagan to the verge of the party’s presidential nod in the latter year), Gingrich did so as a moderate, maybe even liberal. As Ed Kilgore, who was a young Georgia political player in those years, has noted: “Gingrich returned to Georgia and launched his electoral career, running for Congress in 1974 and again in 1976. His incumbent opponent was John Flynt, an old-fashioned conservative Democrat best known for being on the League of Conservation Voters’ “Dirty Dozen” list of environmental reactionaries. Unlike many Georgia Republicans who sought to out-flank Dixiecrats by coming across as better-bred right-wing extremists, Gingrich ran to Flynt’s left, emphasizing environmentalist and “reform” themes, and enlisting significant support from liberal Democrats. Unfortunately for him, these were the two worst election cycles for Georgia Republicans since the 1950s (the Watergate election of 1974 and Jimmy Carter’s Georgia landslide of 1976), and he lost narrowly both times.”

Environmentalist? Appealing to “liberal Democrats”? That was the old Newt Gingrich. He’s a conservative now. Sure, he was kinda green in 1976, but Republican purists can count on Newt now. Right? Well, er, um, he did appear three years ago with former House Speaker Nancy Pelosi in an ad for Al Gore’s Repower American Campaign, an ad that saw Gingrich chirping about how, while he and the liberal Democrat did not agree on many issues, “we do agree our country must take action to address climate change.” That troubled members of a party that has made denial of global warming one of its basic precepts. So Gingrich claimed appearing with Pelosi was “the biggest mistake of his four decades in politics. It wasn’t the biggest mistake. But it was always going to be a disqualifier.

5. GINGRICH CAME UP WITH THE LAMEST EXCUSE EVER FOR CHEATING ON HIS SEVERAL WIVES: To win the Republican nomination, a candidate needs to run well in Iowa and a whole bunch of Southern and Western states where evangelical Christians have been picking winners in caucuses and primaries for decades. These folks are supposed to take infidelity as seriously as they do banning abortion and denying rights to gays and lesbians. And some of them actually do.

So what will they make of the fact that Gingrich is on wife number three, and that he started dating her when he was still with wife number two, and that their affair played out at the same time that he was condemning Bill Clinton for Oval Office hijinks? And what will they think of Gingrich’s excuse? Here, from an interview this year with the Christian Broadcasting Network, is the excuse: “There’s no question at times in my life, partially driven by how passionately I felt about this country, that I worked too hard and things happened in my life that were not appropriate.

You see, it was patriotism—the love of the country, not the love of the ladies—that led him to stray.

That was always going to be a tough sell with those essential evangelical voters in Iowa.

They are ditching him in droves now. The only question is whether the evangelicals will coalesce around a candidate—arguably Santorum or Michele Bachmann—in sufficient numbers to push Gingrich into fourth or fifth place by the time the caucus count is done.

Then he really will be in footnote territory, where, conservatives and liberals ought to be able to agree, this most pompous of political grandstanders has always belonged.