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While they're at it, they also have Bobby Jindal, the Republican governor of Louisiana and perhaps a future presidential candidate, in their sights.
Each man, the birthers say, is ineligible to be president because he runs afoul of the constitutional requirement that a president must be a "natural-born citizen" of the United States.
Rubio's parents were Cuban nationals at the time of his birth, and Jindal's parents were citizens of India.
Alex Leary of the St. Petersburg Times reported that various bright lights of the birther community - Mike Apuzzo, Charles Kerchner, Orly Taitz and Alan Keyes - were casting doubt on Rubio's eligibility.
"Sen. Marco Rubio ... was born a dual citizen of both Cuba and the USA. He is thus not eligible to serve as the president or vice president, Kircher writes in is blog.
There are far more serious impediments in their way - most recently a devastating report by The Washington Post's Manuel Roig-Franzia proving false the central narrative of Rubio's political rise - that he is the son of exiles who fled Cuba under Castro. In fact, his parents left the island, apparently for economic reasons, two and a half years before Castro came to power.
Of course, this isn't a uniquely Republican problem. My colleague Jennifer Rubin, noting a number of anti-Semitic messages seen at Occupy Wall Street events, asked this week, "for respectable politicians and media outlets, where is the outrage?"
The higher prominence of loons of all stripes is a natural consequence of a political system that has lost every last vestige of a political center.
But in the Obama age, this is particularly a problem for Republican lawmakers who are cowed into silence by the fear that any criticism of the crazies will invite a primary challenge.
Now that the birthers have begun to eat their own brightest prospects, perhaps Republican lawmakers will finally feel compelled to say something.
Dana Milbank's email address is danamilbank@washpost.com. He writes for The Washington Post Writers Group.