A new study of late-night political jokes finds the network comedians clearly avoiding humor about Democratic candidate Barack Obama, while piling the jokes on President Bush and Sens. John McCain and Hillary Rodham Clinton.

The study by the Center for Media and Public Affairs finds that only cable’s Comedy Central — whose primary comedians Jon Stewart and Stephen Colbert more closely follow the day’s headlines, which have been dominated by Obama — has slightly more jokes about the freshman Illinois senator.

The study covered all jokes between Jan. 1 and July 31 in late-night monologues by Jay Leno, Conan O’Brien, David Letterman, Stewart and Colbert.

The center found that the network shows broadcast only 169 jokes about Obama, compared with 428 about Bush. McCain drew 328 jokes. Clinton, who dropped out of the presidential race and much political news in early June, still drew more than twice as many attempted yuk lines (382) as Obama.

On Comedy Central, Obama and McCain were close, with 207 and 201, respectively, while Clinton and Bush nearly tied at 179 and 177, respectively.

Combining the laugh lines from all five shows, Bush was the most mocked, with 605; Clinton had 562; McCain got 549; and Obama trailed, with 382.

Letterman had the most fun at Clinton’s expense, with 146 jokes about her and only 46 about Obama. Leno had the most Bush jokes (208), with Clinton right behind at 204.

Colbert worked McCain over the most, with 129 jokes, compared with 91 for Obama and 79 on Clinton. An example: “It’s time the media started trumpeting McCain’s exciting story. He’s old, and no one likes him.”

Obama’s recent Middle East and European tour “may strike some as presumptuous,” said Stewart. “In fact, I joked that Obama would be stopping in Bethlehem to visit the manger where he was born.”