In a brief break from a fierce advertising war, Republican presidential candidate John McCain will air a one-evening-only ad with a simple message for Barack Obama: "Job well done."

The ad will air before, during and after Obama's nomination acceptance speech on national cable television.

In the ad, McCain addresses Obama directly, congratulating him for becoming the Democratic Party's nominee. McCain also recognizes the symbolism of a black man accepting the nomination on the 45th anniversary of the Rev. Martin Luther King Jr.'s "I Have a Dream" speech.

McCain says: "Senator Obama, this is truly a good day for America. 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."

While the ad represents a moment of comity, it also casts McCain as a generous and gracious rival on the final day of the Democratic National Convention where McCain was regularly portrayed in a negative light.

As McCain concedes, this won't last.

Both candidates have been running a series of ads criticizing each other, vastly outnumbering any positive ads about themselves. But this is the first positive ad of the election by one candidate about the other.