Former Republican Sen. Fred Thompson is challenging Martin Sheen in an on-air battle over the war on Iraq.

Thompson - who now plays a top legal eagle on Law & Order -- is starring in a new TV ad to show support for President Bush's hard-line position on Saddam Hussein.

Thompson's new ad counters Sheen, who plays liberal President Josiah Bartlet on The West Wing and has been appearing in an anti-war ad.

"I believe that the president is showing courage in dealing with a situation that is not of his making," Thompson told The New York Post yesterday.

"And for those who ask what has Saddam done to us, I ask what had the 9/11 hijackers done to us before 9/11?"

Thompson joined the cast of Law & Order this season in the character of a district attorney. Both shows air back-to-back on NBC Wednesday nights.

The battle -- which was first reported by Internet gossip Matt Drudge -- pits a former real-life senator playing a make-believe DA against a fictional president portrayed by a political-activist actor.

"I've been intent that I stay involved with the issues that were important to me even after I left the Senate," Thompson said. "You don't have to be in elected office to have input."

Thompson, a lawyer, served in the Senate from 1994 until this year, but has been acting in movies and on TV since the mid-1980s.

The ad is expected to start airing on Monday in New York and Washington.

"Mr. Sheen and Mr. Thompson are both acting in their capacity as private citizens," NBC spokeswoman Rebecca Marks said yesterday. "We respect their opinions and their right to free speech."

Sheen's ad promotes a "virtual march" -- sending anti-war e-mails and faxes to the White House - to oppose military action against Iraq.

To bypass skittish networks, it aired this week in Los Angeles and Washington on local cable channels during programming on Fox News Channel and CNN.