Bush's campaign is trying out a new line of attack. (Photo by Bill Pugliano/Getty Images)

One-time Republican front-runner Jeb Bush has gotten as much traction against Donald Trump as an eel navigating an ice rink. His official campaign, and the Right to Rise super PAC working independently to elect him, have traded neglect for scorn. The latest example came after Trump's interview with Mark Halperin, an interviewer with an uncanny ability to push into new territory. He asked Trump who he might support for the Supreme Court. Trump dodged.

"What about your sister?" asked Halperin.

"My sister's great," said Trump. "I have a sister who's on the court of appeals."

"She'd be a good Supreme Court justice?" asked Halperin.

"I think she'd be phenomenal," said Trump. "I think she'd be one of the best. But frankly, we'd have to rule that out."

That quote ran on Aug. 26. One day later, National Review columnist Ramesh Ponnuru pointed out that Maryanne Trump Barry was reliably pro-choice, and once rejected a lawsuit to stop partial birth abortions for "semantic machinations" about when life began. Just 20 minutes after that article went up, Bush's spokesman and campaign manager tweeted it out, sexing it up a bit to say that Trump actually wanted to put his sister on the bench.

Trump has demonstrated an astounding ability to survive statements that would have been hobbling gaffes for other candidates. This statement might not even qualify. The typical Supreme Court question for a presidential candidate is not "who would you put on the court," but "who is your model nominee." A Republican needs just six syllables to ace that question: An-to-nin Sca-li-a. It's vanishingly rare to hear a candidate asked who, specifically, he would nominate; saying so would open some relatively unexamined judge up to media vetting. Trump's answer was basically a brush-off. (At 78, Maryanne Trump Barry is at least 20 years older than the sort of nominees parties now prefer.)

Still, the delayed reaction attack from Bush's team highlighted the vulnerabilities that Republicans see for Trump. He's surged in a vacuum, without ever being subjected to the sort of mail, radio and TV attacks that rival campaigns are saving up for. While Sen. Rand Paul's campaign produced an online video of old, liberal-sounding Trump positions -- not even including a 1999 interview where he called himself "pro-choice" -- no one has put money behind such an attack.

Trump, with his habit of self-contradicting within a single paragraph of speech, may be especially vulnerable to attacks on his old heresies. So far, the Bush network is not quite sure how to execute those attacks. On April 20, Right to Rise chief strategist Mike Murphy told The Washington Post that the super PAC would not "uncork" money to beat Trump. "Trump is, frankly, other people’s problem," he said. One day later, Right to Rise paid for a plane to buzz around Trump's rally in Mobile, Alabama, telling onlookers that he supported "higher taxes."

UPDATE: At 1:28 p.m., Concerned Women Political Action Committee CEO Penny Nance issued a statement condemning Trump's praise for his sister.

"Donald Trump's statement that his pro-partial-birth abortion sister, federal judge Maryanne Trump Barry, would make a 'phenomenal' Supreme Court Justice is alarming and disturbing," wrote Nance. "While we all love our families, a president makes Supreme Court nominations that affect us all. Past presidents have erred in their appointments. That Mr. Trump can so easily offer effusive praise of a judge who is in favor of giving constitutional protection to partial-birth abortion - simply based on the fact that the judge is his sibling - calls into question not just his readiness to be the nominee for the party that fights for the unborn, but his overall fitness for the most important job in the world."