There is nothing wrong with a think tank inviting a presidential contender to give a speech. There is nothing wrong with conducting a Q&A at that speech. However, when an esteemed think tank brings in a candidate, sets up ground rules to shield the candidate from tough questions, redirects a question to an adviser and then intervenes to protect the candidate from an embarrassing query, a line has been crossed.

That’s essentially what happened at the American Enterprise Institute’s program with Herman Cain this morning. For 25 minutes, Kevin Hassett of AEI asked a series of softball, open-ended questions. We started off with a lengthy discussion on pizza crust and Cain’s experience at Godfather’s Pizza. The questions were on 9-9-9, including such probing inquiries such as whether Cain was surprised at his opponents’ reaction to his tax plan. There were no follow-up questions. Cain wasn’t asked to respond to obvious critiques of the 9-9-9 plan. There was no challenge, for example, to his claim that his VAT promotes exports or that the plan would hike taxes on the poor and middle class.

A brief breath of fresh air occurred when AEI’s Jim Pethokoukis asked a hard question from an Internet discussion on double taxation of labor. Cain looked stunned. That inquiry was promptly shuffled off to a Cain adviser. Hassett chimed in that those “technical” question should go to aides. How helpful!

It is a shame Pethokoukis did not moderate the entire session. We might have learned something, and AEI might have avoided appearing unduly solicitous of a presidential candidate.

In the Q&A, Jon Karl of ABC broke the ground rules and asked about the sexual-harassment allegations made against Cain by two women. Cain said he was following AEI’s ground rules. Hassett jumped in to say Cain would be at the National Press Club later. Again, AEI to the rescue.

AEI would have been better advised, if it intended not to ask any probing questions of Cain, to have let him give a speech, as New Jersey Gov. Chris Christie did this year. An opportunity was lost to ask tough (polite, but tough) questions about a tax plan that has engendered much criticism.