BY DANIEL RUTH

When he eventually gets around to penning his memoir, “Lemming of the Senate,” perhaps Sen. Marco Rubio will finally answer the most vexing question regarding his career.

Why would a hustling politician go to all the trouble to run for and win one of the most coveted jobs in American politics, only to spend his time as a cowering apologist for the Sammy Glick of the West Wing?

You would think for the $174,000-a-year one gets for the part-time job of U.S. senator, every now and then Rubio might actually stand for something, anything, remotely resembling a principle. Too heavy a lift?

Rubio, whose forthcoming memoir, “Shadows That Scare the Bejabbers Out of Me,” was in fine form a few days ago when he was asked if he might have just the merest reservation that President Donald Trump had encouraged China to help dig up dirt on a potential political rival, former Vice President Joe Biden,.and Biden’s son, Hunter.

In highly technical legal terms, soliciting a foreign power to become your bagman is considered really, really stupid. And illegal.

But not to Master Marco, whose forthcoming memoir ought to be titled, “Profiles In Obsequiousness.” Rubio had no problem with Trump trying to enlist China’s Xi Jinping as his Sancho Panza of corruption.

Indeed, Rubio, R-Boo!, blithely dismissed Trump’s efforts to extort a foreign power to do his knee-capping as little more than a presidential exercise in hilarious comedy.

“He’s pretty good at getting everybody fired up and he’s been doing that for a while and the media responded right on task,” Rubio shrugged.

And he’s quite right. The media does indeed get fired up by a sitting president extorting foreign leaders and subverting the nation’s electoral processes. Yes, it is a professional character flaw. Sorry about that.

The problem here is that Trump is hardly the Henny Youngman of the Beltway. Nobody has ever confused Trump’s foaming blustering with comedy. Belittling Gold Star families, or physically disabled reporters, or referring to ca-ca countries, or playing footsie with Vladimir Putin, or throwing the Kurds under the bus, does not quite qualify as open mike night at Side Splitters.

Indeed, with the possible exception of Andrew Jackson’s knee-slapping genocidal Trail of Tears, Trump may well be the most humor-challenged president in American history.

Rubio dismissing a plea to enlist the help of China to hold on to power as the handiwork of a jesting Trump allowed the senator to avoid the distasteful prospect of exhibiting some scruples and incurring the president’s ire.

Oh sure, Rubio, no doubt hard at work on his forthcoming memoir: “A Man For All Alibis,” could point to his hard-hitting criticisms of China as proof positive of his tough guy bona fides.

But really now, isn’t casting stink eyes in the general direction of Chinese oppression and duplicity a bit like coming out foursquare in opposition to bank robbery, which indeed is bad, very bad.

Rubio, R-Shrinking Violet, needs a civics lesson. So does Trump. But that is a whole other story.

Upon ascending to the U.S. Senate, Rubio took an oath of office swearing to protect and defend the United States against enemies foreign and domestic. He pledged his fealty to the U.S. Constitution – not to Trump, or Senate Majority Leader Mitch McConnell, or the Republican Party, or Sean Hannity, or Fox News.

To make this commitment a bit easier, Rubio, whose forthcoming memoir ought to be titled “Women And Children First, But Not Before Marco Rubio,” isn’t even up for re-election until 2022. But apparently even three years away from facing the voters isn’t enough time to grow a spine or find a conscience.

Florida’s senior senator could have done the right thing. He could have at long last found a moral voice to decry a sitting president’s ham-handed grifting of Ukraine and China to provide opposition research to further his political agenda.

He could have committed (shush!) leadership. Ah at last! A real spit-take punchline.

The Constitution, so quaint, so charming, is always the first casualty of political ambition. The nation’s founding document becomes so much confetti when craven glad-handers can’t bring themselves to stand up for the very country they swore to defend.

And great gales of laughter ensued.