Be careful who you throw under the campaign bus.

Mitt Romney’s backers decided to sully U.S. Supreme Court Justice Sonia Sotomayor’s good name. It was one of their latest jabs against GOP opponent Rick Santorum — and another craven attempt to prove their candidate is the “true conservative” in the race.

The Romney radio ad, debuted in the run-up to Super Tuesday, criticizes Santorum’s 1998 vote to confirm Sotomayor to the Second Circuit Court of Appeals during his time in the Senate. That put her one step away from infiltrating the highest court with her liberal activist ways, the ad suggests, where she can create havoc for conservative causes. Who knew it was all Santorum’s fault?

The ad’s got problems.

First, Santorum is without a doubt the most genuinely conservative candidate in the presidential primary race. He’s not bluffing when he speaks of feeling queasy about the separation of church and state.

By any honest accounting, Romney is the most moderate of the remaining Republican candidates, although he is furiously covering his tracks in his quest to win delegates. His middle-of-the-road record would give him the best chances in the general election, but it counts against him among the Republican faithful.

In crafting the ad, Romney’s folks were banking on the listener’s limited knowledge. Yes, it was Barack Obama who appointed Sotomayor to the U.S. Supreme Court. But it was George H. W. Bush who launched her judicial career by appointing her to the District Court for the Southern District of New York in 1991. (She was confirmed in 1992.)

Details, details.

Jay Sekulow, a leading conservative lawyer, is the voice of the ad. He notes that Santorum approved Sotomayor while “29 of his Republican colleagues voted against her.”

True. And 25 Senate Republicans supported her. Santorum wasn’t the only one.

“With Mitt Romney, we know what kind of judges we will get,” Sekulow concludes.

Sometimes you do know what you’ll get when it comes to judicial candidates, and conservatives and liberals alike wish for litmus tests.

However, with Sotomayor, no one had a crystal ball during her confirmation. She’s a bit of a judicial enigma. Her stands on some of the hot-button social issues are unknown.

She won praise from Planned Parenthood and gay and lesbian advocates, but they were hedging their bets.

Sotomayor’s only previous ruling that dealt with abortion was one where she sided with the pro-life arguments. The case involved a policy that prohibited giving government funding to groups that either perform or advocate for abortion in foreign countries.

She never directly put her judicial view on the core issue, the constitutionality of abortion.

Meanwhile, she’s been very clear about how she views the Constitution. This statement, from her 1997 confirmation hearing, could be used in a political ad positioning her as the patroness of conservative causes, not the devil’s mistress: “I don’t believe we should bend the Constitution under any circumstance. It says what it says. We should do honor to it.”

Interestingly, Sotomayor exemplifies a character trait that Romney sorely lacks: the emotional intelligence to read social class. She’s had to learn and negotiate the divergent protocols and unwritten social codes of her childhood neighborhood in the Bronx, of her parochial high school, of Princeton and Yale law school, and then of private law practice and the federal bench.

Unlike Romney, Sotomayor wasn’t raised the entitled child of a business executive and governor. She was raised by a widow and lived in public housing.

No one can change the facts of their birth. Nevertheless, Romney’s stiffness and many gaffes peg him as an elitist par excellence, giving voters reason to wonder if he can relate to people born to less privilege.

In the caucuses so far, he has done one very well with upper-crust voters, people who maybe could make a $10,000 bet, as Romney proposed during an early debate.

It’s possible that campaign flatulence like the anti-Sotomayor ad helped put Romney over the top in Ohio. After all, you can fool some of the people some of the time. But by maligning a public figure with real depth and complexity, Romney’s operatives invited thinking people to ponder the essentially hollow character of the Republican frontrunner.

Good luck with that strategy in November.