'Sloppy looking . . . hack . . . bad person . . . so-called pundit."

What sin prompted these classy insults from Donald Trump? I objected to him moderating a televised Republican presidential debate. Originally scheduled for Dec. 27, it has now been canceled (after only Newt Gingrich and Rick Santorum had agreed to participate).

When the debate was announced, I suggested Mr. Trump was unlikely to be an impartial questioner. He had already said he would be "probably endorsing somebody right after" the debate and was already "leaning" toward one candidate. He had also threatened to run for president as a third-party candidate. It would be folly, then, for the GOP to lend credibility to a prospective spoiler who, if he entered the race, would split the anti-Obama vote.

I added that it wasn't wise for Republican presidential hopefuls to associate with someone who began his own (aborted) bid for the GOP nomination by declaring Barack Obama ineligible to be president because he wasn't born in the United States—an opinion he still holds today.

Mr. Trump's reaction simply reinforced my points. But this kerfuffle obscures larger questions about the merits and shortcomings of this year's GOP debates. A dozen have been held so far this year, with another being hosted Thursday night by Fox News.

Watch the GOP debate live from Iowa at 9 p.m. ET on Fox News Channel and streaming live on FoxNews.com from 8:30 - 11:30 p.m. ET.

On the plus side, the debates have allowed every potentially serious candidate to be seen by large audiences (an average of 4.5 million people have tuned in to each one). They have helped candidates sharpen sound bites and flesh out images. And they've kept alive candidacies that might have otherwise died due to lack of interest.

For the most part, the debates have been helpful. Before them, the "generic Republican" never led President Barack Obama in any Gallup survey. Since early July, the generic GOPer has often been leading Mr. Obama. The debates likely contributed to this shift.

Karl Rove is a former senior adviser and deputy chief of staff to President George W. Bush. He is a Fox News contributor and author of "Courage and Consequence" (Threshold Editions, 2010). To continue reading his column in The Wall Street Journal, click here.