There are two swift routes to political downfall. One is sex. The other is money. The first is humiliating but survivable. The second tends to be terminal, even criminal.

Today’s topic is the second, in the form of Virginia Gov. Bob McDonnell (R) and the now mountainous evidence that — whether he technically complied with Virginia’s Swiss cheese disclosure laws or not in accepting thousands of dollars in gifts from a wealthy businessman — he has no business continuing in office.

The sordid McDonnell details in a bit, but first the comparisons between politicians and illicit sex and politicians and illicit money. They are linked to the twin delusions of the erring politician: his (I use the male form intentionally) sense of entitlement and his conviction of invulnerability.

I work so hard, the politician tells himself. I deserve a little (insert specific failing).

No one will find out, the politician tells himself. I was smart enough to get elected (governor/president/senator).

Wrong, wrong, wrong.

There are differences, as well, between the politician tripped up by sex and the one felled by greed. The former can argue that he was not thinking with . . . well, he was not thinking. He is hardly the first to do something dumb in the grip of lust, love, whatever.

Yet he most likely has a wife and family, collateral damage in his sexual escapades. Points off for that — and more off if he has his wife by his side at the confessional news conference.

The greedy pol is blameworthy in a different way, again both heightening and lessening his guilt. On the negative side, he was not swept away by the passion of the moment; he calculated that he could accept the money, the Rolex, whatever, and get away with it.

On the plus side — and this is explanation, not excuse — he may have been acting under familial pressure, and in what he conceived as the best interests of his family, rather than against it, as the straying spouse certainly has.

Much modern political corruption, especially of the penny-ante sort, can be explained by the yawning gap between the relatively paltry income of the politician and the wealth of the private-sector types fluttering around him.

The politician feels aggrieved, which in turn feeds his sense of entitlement. The political spouse sees her friends driving fancier cars, wearing fancier clothes — all this while her husband is probably working longer hours, to the detriment of his family. You can understand, although not excuse, the husband whose ethical judgment is warped by marital guilt, the wife whose judgment is warped by marital resentment.

Which brings us to the McDonnells, and the flagrant, repeated misconduct exposed by The Post’s Rosalind Helderman. The story began with relatively trivial, if astonishingly morally obtuse, bits of graft and back-scratching:

The $15,000 check that businessman Jonnie R. Williams Sr. gave to help cover the catering bill at the McDonnells’ daughter’s wedding — an event that took place three days after Virginia first lady Maureen McDonnell flew to Florida, where she touted a dietary supplement made by Williams’s company, Star Scientific Inc. Three months later, Star Scientific used the governor’s mansion for a luncheon, attended by the governor, to promote the supplement.

The $6,500 Rolex, complete with engraved inscription, “71st Governor of Virginia,” that Williams bought for the governor at Maureen McDonnell’s behest. She allegedly requested the bauble moments before a meeting she had arranged for Williams to pitch a top state health official on the supplement.

● Maureen McDonnell’s reported $15,000 spree at Bergdorf Goodman, again on Williams’s tab — this a year after a staffer foiled McDonnell’s bid for a Williams-underwritten Oscar de la Renta inaugural gown.

Now comes reporting that raises the story to a new level of outrage: Williams last year gave $70,000 — supposedly a loan — to a corporation owned by McDonnell and his sister; plus $50,000 to Maureen McDonnell in 2011, and $10,000 as a wedding present this year to another McDonnell daughter.

As astonishing is the governor’s technocratic defense: that he is complying with the letter of Virginia disclosure rules, which do not require reporting of gifts to family members. “To, after the fact, impose some new requirements on an official,” McDonnell told a Norfolk radio show, “obviously wouldn’t be fair.”

But gifts and entanglements like these are simply wrong, a violation of the governor’s duty to citizens, whatever the rules. That McDonnell doesn’t get this basic point makes him unfit for office. Obviously.

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