On a conference call this morning, two consecutive reporters pressed senior Mitt Romney adviser Russ Schriefer on why the campaign won't reveal its so-called mystery speaker in tonight's convention lineup. Fox has reported that according to a GOP source, it will be Clint Eastwood.

Schriefer's response: "Because if it was a mystery speaker, it wouldn't be a mystery anymore."

That's all Schriefer would say, declining to even confirm whether or not any "mystery speaker" would indeed speak.

Speculation has swirled around the appearance of a mystery speaker at the GOP convention late in the program tonight. The lineup is slated to include speeches from Newt and Callista Gingrich, former Florida Gov. Jeb Bush, Florida Sen. Marco Rubio, and Romney.