Editor's note: Ruben Navarrette Jr. is a CNN.com contributor and a nationally syndicated columnist.
San Diego, California (CNN) -- Is Marco Rubio auditioning for vice president -- or secretary of State?
The Florida senator -- a Republican Party rock star who has been entrusted with the Herculean task of pulling the GOP out of the hole that it has dug with Latino voters because of harsh rhetoric on immigration -- delivered a substantive speech on U.S. foreign policy Wednesday at the Brookings Institution.
It was an ambitious subject choice. At the moment, Rubio could read the Miami-Dade phone book and he'd get his share of media attention. A speech like this suggests he's trying to prove something -- or perhaps prove himself worthy of all the attention he is already receiving.
As for the speech itself, Rubio's message was right on target. He stressed the idea that the United States remains what former Secretary of State Madeleine Albright called the world's indispensable nation.
"On the most difficult transnational challenges of our time," Rubio asked, "who will lead if we do not? The answer, at least today, is that no other nation or organization is willing or able to do so."
You would think that, for some folks in my profession, Rubio is the indispensable politician. What would we do without him? Any story involving him is bound to get a lot of attention.
As speculation builds that presumptive GOP nominee Mitt Romney has his eye on the charismatic 40-year-old Cuban-American as a possible running mate, political observers want to learn everything they can about him.
Some of what we're learning could be embarrassing to Rubio, like the tidbit that surfaced this week about his maternal grandfather. The story appears in a forthcoming and unauthorized biography of the senator by Manuel Roig-Franzia of the Washington Post.
Rubio was born to Cuban exiles in Miami in 1971, and so he's automatically a U.S. citizen. But, according to excerpts from the book, which is due out in June, his grandfather, Pedro Victor Garcia, entered the United States without a visa in 1962 and spent the next few years in legal limbo while pleading for leniency from U.S. immigration officials. Finally, Garcia was ordered to leave the United States.
Normally, this kind of family history would hardly be worth mentioning. And, while I expect that some right-wing talk show hosts will suggest that the reason the media is mentioning it now is to try to sour conservatives on Rubio, that's really not what's going on here. Conservatives aren't likely to hold Rubio accountable for the actions of his grandfather.
This story is important because it drives a wedge between Rubio and the voters he is supposed to be able to deliver to the Republican Party. When you're a Cuban-American politician who is being put forth by your party to help get votes from Latino voters -- the majority of whom are Mexican or Mexican-American -- things can get complicated.
When it comes to immigrating to the United States, Cubans get preferred status. Thanks to the Cuban Adjustment Act, which was enacted in 1966 -- or four years after Rubio's grandfather came to the United States -- Cuban refugees who flee the Island and reach the U.S. shoreline have a clear path to legal residency and eventual citizenship.
Mexican immigrants aren't so fortunate. So when Cuban-Americans do what Rubio has done since arriving in the Senate 16 months ago and take a hardline against illegal immigration, Mexicans and Mexican-Americans have been known to cringe. After all, that's easy for them to say.
Last year, Rubio came under criticism from fellow Latinos for getting wrong the year that his parents came to United States. It turned out that they left Cuba before Castro took power, making them immigrants and not refugees.
So here we have one more thing that Mexicans and Mexican-Americans can use against Rubio to try to take him down a peg or two. They'll gladly remind him that he's no better than they are, and that while some of their ancestors may have come to this country illegally, so did at least one of his.
As bad as this relationship has been, it's about to get worse because of "grandpa-gate." Stories like this make Rubio damaged goods and less useful in luring Latinos to the Republican ticket. That being the case, his vice presidential stock has to be plummeting. What good does it do the ticket for Rubio to be popular with whites and Cuban-Americans? Republicans are likely to get the majority of those votes anyway. His value is all wrapped up in how well he plays with Mexicans and Mexican-Americans. And right now, the answer is "not well."
Marco Rubio on the GOP ticket? I wouldn't bet on it. Say, maybe secretary of State would be a good consolation prize after all.
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The opinions expressed in this commentary are solely those of Ruben Navarrette.