Poor Marco Rubio.

As the prospects for comprehensive immigration reform sink, so go his hopes of establishing himself as the solid Republican frontrunner in the 2016 campaign for the White House.

Meanwhile, the junior Florida senator is under siege from the bug-eyed right wing of his own party. Glenn Beck called him a “piece of garbage,” and even the Tea Party has turned on him. It’s gotten so bad that GOP action groups are putting out commercials saying nice things about Rubio, just to preserve his shot at the presidency.

Unfortunately, immigration reform is the only serious issue on which Rubio has presumed to lead. Otherwise, his time in Washington has been quiet and forgettable.

During the big post-Newtown debate on expanding background checks of firearms buyers, Rubio revealed himself as just another gutless sniveler controlled by the NRA. In the budget battle he offered not a single new idea, only boilerplate attacks on President Obama over the federal deficit (which is now, to the chagrin of Republican presidential hopefuls, shrinking).

Immigration reform was to be Rubio’s golden ticket to the nomination — a young Hispanic candidate from a critical swing state, bridging with Latino voters a huge gap that helped cost Mitt Romney the election last year.

The immigration bill that has finally passed the Senate would add more resources for border security while offering a long road to full citizenship for millions of illegal immigrants. The legislation is doomed to crash in the Republican-controlled House of Representatives, where Speaker John Boehner has been neutered by the hardcore who take their cues from radio screamers like Beck.

Many of those House members disdainful of immigration reform don’t have to worry about their own re-election because they come from carefully gerrymandered districts where the majority of voters are older white conservatives.

As long as the House remains tilted so far right of the nation’s political center, and continues to smother all efforts at moderate compromise, the Republicans have virtually no prayer of recapturing the White House in three years.

This grim obstacle has become clear to Rubio and others seeking to be the next GOP nominee, as well as to some heavy political action groups that have launched an unusual ad campaign in several states.

One Florida ad running on Fox News encourages viewers to phone Rubio and “thank him for keeping his promise, and fighting to secure the border.” The commercial was funded by the conservative American Action Network (these big-money groups always have the word “American” in their name, to show how patriotically unselfish they are).

Another one, Americans for Conservative Direction, recently ran pro-Rubio ads in Iowa, the first major primary state, and also the whitest. “Stand with Marco Rubio to end de facto amnesty,” the commercial proclaimed.

And next month, in one of the grandest hypocrisies of the entire immigration furor, the Americans for Prosperity Foundation is for the first time taking its annual conference away from Washington.

The new site: Orlando. The keynote speaker: Sen. Marco Rubio.

Why is this so funny? Because the Americans for Prosperity Foundation is basically the infamous Koch brothers, Charles and David, those ultra-conservative billionaires who spend their free time and money trying to buy elections.

Paradoxically, their campaign contributions and massive media blitzes helped to install some of the same fire-breathing gasbags in Congress who are now dismantling immigration reform and damaging Rubio’s chances to be president.

That the Kochs would come to Florida and put Rubio center stage illustrates the bewildered desperation now plaguing the Republican Party. Charlie and Dave have seen the sorry poll numbers from 2012, and know they can’t win the White House without a titanic shift of Hispanic votes.

Apparently the strategy is to present a candidate who is heroically identified with pushing for immigration reform, while the brothers continue working backstage to ensure that reform itself has zero chance of becoming law.

Maybe that’s the secret strategy of the GOP leadership, too. The recent burst of political ads isn’t a pro-immigrant pathway so much as pro-Rubio, portraying him as a principled crusader on a sensitive issue.

The aim is to build him up as presidential material and deflect the ridicule from the far right.

For a candidate comfortably positioned in the political mainstream, being called “a piece of garbage” by a clown like Glenn Beck would be a badge of honor, something to brag about.

Rubio’s problem is that he isn’t in the mainstream, and he doesn’t have the conviction to get there. He won’t stand up to Beck just like he wouldn’t stand up to the NRA.

And if the immigration overhaul goes down the tubes, he might be standing in the wings at the next Republican convention, watching someone else get nominated.

(Carl Hiaasen is a columnist for The Miami Herald. Readers may write to him at: 1 Herald Plaza, Miami, Fla., 33132.)

Photo: Gage Skidmore via Flickr.com