These are serious times for our country — from challenges abroad, to our crippled economy, to the tragedy in the Gulf of Mexico. With events filtered through a partisan midterm election and a heat wave smothering the nation's capital, it seems the first thing to wilt is Washington's sense of humor. Jon Stewart, David Letterman, Jay Leno and the rest of the late-night crowd continue to hold their own. But the ability of elected officials to indulge in a good collective laugh — not to make light of serious situations but to ease tensions — has become a too-rare civility……