DAYTON, Ohio  The Obama campaign was already nervous that Senator John McCain might try to steal the show from their big night by letting the name of his running mate selection slip out, and when the McCain campaign announced Thursday that it would run a new television commercial during their convention, those fears intensified.

But when it was finally broadcast, the television advertisement that they had feared would be a knife in their back turned out to be something of a Valentine.

“Senator Obama, this is truly a good day for America,” Mr. McCain said, talking straight to the camera, in a striking departure from some of the pointed advertisements he has run of late. “Too often the achievements of our opponents go unnoticed. So I wanted to stop and say, Congratulations. How perfect that your nomination would come on this historic day. Tomorrow, we’ll be back at it. But tonight, Senator, job well done.”

It was just a blip in a day of fevered speculation about just whom Mr. McCain would tap as his running mate, a day in which there were tea leaves to read everywhere but few hard and fast facts. McCain campaign officials said the campaign had no intention of letting their closely held secret out Thursday.