Republican presidential hopeful Mitt Romney was already polling at historically low numbers among Hispanic voters before his decision to name Wisconsin Rep. Paul Ryan as his running mate. Now, Romney risks a total debacle among Hispanic voters that could cost him the election.

There are five major reasons why Ryan, a darling of the Tea Party who is known for his budget-cutting proposals and hard-line stands on immigration in recent years, will have a hard time helping Romney solve his Latino vote problem.

First, Ryan is not Marco Rubio, the Spanish-speaking Cuban-American senator from Florida, nor Rob Portman, the Ohio senator and former U.S. trade representative who also speaks fluent Spanish and was on Romney’s short list for vice-presidential nominee.

Either Rubio or Portman could have helped Romney win key swing states that will be crucial to win the November elections and could have campaigned nationwide speaking in Spanish to Latino audiences. Ryan, who comes from a state with only 5 percent of Latino voters, has zero connection with Hispanic voters.

While the Romney campaign has announced that it will give Republican Hispanic stars such as Rubio stellar roles at the upcoming Republican convention in Tampa — but not the keynote speech, which has been awarded to Gov. Chris Christie of New Jersey — Romney is in deep trouble with Hispanics.

According to a June USA Today/Gallup poll of Latino voters, President Obama was already beating Romney 66 percent to 25 percent among registered Latino voters. That’s the lowest any Republican candidate has polled among Hispanics in 16 years.

Former Republican candidate Sen. John McCain won 31 percent of the Hispanic vote in 2008, and former President George W. Bush won 40 percent in 2004.

Second, although most Hispanic voters are young and don’t care that much about health care, the Obama campaign ads depicting Ryan as a ruthless budget cutter who would slash Medicare and Medicaid is likely to hurt the Republican ticket among Hispanics in Florida, which has a huge population of Hispanic senior citizens. (Ryan’s past opposition to the U.S. embargo on Cuba, since reversed, won’t help him either among Florida’s Hispanic seniors.)

Third, unlike Republican moderates, Ryan strongly opposes a path to legal residence for the estimated 11-million undocumented U.S. residents. Most Hispanics support a path to citizenship for undocumented residents who have long lived in this country and have no criminal records.

“I do not support amnesty for the millions of illegal immigrants already living in the United States,’’ Ryan says on his Congressional website. He does not elaborate on what he would do about them, which suggests he may support Romney’s formula of “self-deportation,” a proposal that Latino leaders say amounts to making the life of undocumented residents so miserable that they would leave voluntarily.

Fourth, Ryan has opposed the DREAM Act, a bill overwhelmingly supported by Hispanics — and many non-Latinos, too — that calls for giving a path to residency to up to 1.7 million young undocumented students who were brought to the United States as infants and raised in this country.

Many of them don’t even speak the language of their native countries. While most of them were recently granted de facto temporary residency by Obama, their legal status remains in limbo in the long run.

Fifth, Ryan voted in 2005 in support of the Sensenbrenner bill, sponsored by fellow Wisconsin Republican Rep. James Sensenbrenner. Critics described the bill as the most draconian anti-immigration proposal ever. It passed the House, but died in the Senate.

Romney campaign officials say none of this will matter much on Election Day, because Hispanics’ main concern will be the economy — an area where they hope Romney will enjoy a huge advantage.

In addition, they say that nationwide polls of Hispanics won’t mean much, because the election will be decided in a handful of swing states, such as Florida, Ohio and Colorado, where Romney polls better among Hispanics than in Democratic-controlled states with huge Hispanic populations such as California or New York.

My opinion: Romney’s pick for vice-presidential nominee, coming shortly after Romney’s foreign policy tour in which he traveled to Europe and Israel without even making a symbolic stop in Mexico, will make a bad situation worse for the Republican campaign. It suggests that Romney has thrown in the towel on the Hispanic vote. That may turn out to be the biggest mistake of his campaign — and a godsend for Obama.