It is always difficult to pinpoint the precise moment when a politician reaches his “sell-by” date.

We have seen some remarkable comebacks in America.

Richard Nixon, defeated for president and then governor of California, returned to win the presidency — and then when he was forced from the White House repositioned himself as something of an elder-statesman.

Joe Biden, forced out of the presidential race of 1988 and then defeated decisively in the presidential race of 2008, finally got to the White House in 2009 as the vice president.

So a lot of things are possible.

But it seems safe to say that New Gingrich has nothing left to add to the discussion.

The truth is that Gingrich may have been finished a long time ago.

Forced to resign the speakership of the House in the late 1990s after steering both his party and his personal life on disastrous courses, he has struggled to renew his franchise as the Grand Old Party’s big thinker.

There’s no question that the party of “no” needs a senior figure who is capable of wrestling with ideas. Unless Republicans plan to nominate Dick Cheney, Rush Limbaugh or Michael Steele for president in 2012 — and even the craziest of the crazies are not that crazy — they are going to need someone with a vision. And even if his vision is flawed, Gingrich’s ability to grab forums and to use them for something more than mere grumbling seemed to be getting him back in the running.

But Gingrich tweeted off the deep end Wednesday when he jumped over the cliff of responsible Republicanism and into the chasm of right-wing talk-radio delusion.

Responding via Twitter to the nomination of Federal Appeals Court Judge Sonia Sotomayor to fill the vacancy that will be created by the retirement of Justice David Souter, Gingrich did not think. He took his talking points from Limbaugh, who has been trying to foster the fantasy that the nominee is some kind of “reverse racist.”

At issue are comments made by Judge Sotomayor’s when she delivered the Judge Mario G. Olmos Memorial Lecture in 2001 at the University of California, Berkeley, School of Law.

The judge took as her topic, “Raising the Bar: Latino and Latina Presence in the Judiciary and the Struggle for Representation.” And she explored the subject at great length, employing statistics, historical references, reflections on her own experiences as a lawyer and judge and a serious consideration of questions of bias and responsibility when it comes to seeking justice.

Unfortunately for her, and for rational discourse, Limbaugh — and by extension Gingrich — focused on one sentence fragment. The judge allowed, toward the end of her address, as how, “I would hope that a wise Latina woman with the richness of her experiences would more often than not reach a better conclusion than a white male who hasn’t lived that life.”

Limbaugh labeled that statement “racist,” suggesting that Judge Sotomayor had dismissed white men as inferior beings. That the radio host would hear such an implication says a great deal about him and very little about President Obama’s first nominee to the high court.

But it was enough for Gingrich.

On Wednesday, the former Speaker of the United States House of Representatives tweeted: “Imagine a judicial nominee said ‘my experience as a white man makes me better than a latina woman.’ new racism is no better than old racism.”

A few minutes later, he followed up with the message: “White man racist nominee would be forced to withdraw. Latina woman racist should also withdraw.”

Aside from the caveman-quality of his calculus, Gingrich is wrong.

Really wrong.

Had Gingrich bothered to read Judge Sotomayor’s speech, rather than simply take a talking point from Limbaugh, he would have quickly realized that the jurist was ruminating on how to assure that biases do not unfairly influence decisions or undermine justice.

“Each day on the bench I learn something new about the judicial process and about being a professional Latina woman in a world that sometimes looks at me with suspicion,” Judge Sotomayor explained at the conclusion of the address. “I am reminded each day that I render decisions that affect people concretely and that I owe them constant and complete vigilance in checking my assumptions, presumptions and perspectives and ensuring that to the extent that my limited abilities and capabilities permit me, that I reevaluate them and change as circumstances and cases before me requires. I can and do aspire to be greater than the sum total of my experiences but I accept my limitations. I willingly accept that we who judge must not deny the differences resulting from experience and heritage but attempt, as the Supreme Court suggests, continuously to judge when those opinions, sympathies and prejudices are appropriate.”

That’s not a racist speaking. That’s not some judicial outlier. That is a responsible — and genuinely mainstream — thinker on judicial matters.

Newt Gingrich’s tweets mark him as the outlier.

Instead of calling for Judge Sotomayor to withdraw her nomination, the former speaker might want to consider whether it time for him to withdraw from the public debate he diminishes and dishonors by repeating the crude fantasies of a talk-radio ranter.