WASHINGTON -- The closely watched San Joaquin Valley congressional race between Rep. Jeff Denham, R-Calif., and challenger Jose Hernandez has veered again into court, with the incumbent suing Democratic operatives for defamation.

Angry over what he calls a misleading political ad, Denham filed suit late Thursday against Sacramento television stations and the Democratic Congressional Campaign Committee. The unusual campaign-season libel suit came after Democrats and the TV stations declined to pull the 30-second spot.

“Not only is the ad a flat-out lie, but it’s malicious, and they should be held accountable,” Denham said in a telephone interview Friday.

The ad, at the very least, is selective in its use of facts. It accuses Denham, an Air Force veteran, of refusing to guarantee salaries for the military in the event of a government shutdown last year while opposing a Democratic measure to stop congressional pay during a shutdown.

Democrats are standing by the ad.

“No matter how hard Congressman Denham tries to cover up his record and distract from his votes, he voted to protect his pay and not the pay of our troops,” Jesse Ferguson, the communications director of the Democratic Congressional Campaign Committee, said Friday, repeating the gist of the disputed ad.

Legally, Denham faces high hurdles. As an elected official, the quintessential public figure, Denham must demonstrate that the statements about him were made by individuals who knew the assertions were false or who acted with reckless disregard for whether they were false. Judges often give wide leeway to heated and even misleading political speech, in particular.

“Debate on public issues should be uninhibited, robust and wide-open,” the Supreme Court noted in a landmark 1964 free-speech case, adding that “erroneous statement is inevitable in free debate.”

Former Valley Congressman Gary Condit, for instance, struggled to prevail in court when he filed multiple defamation lawsuits against tabloid newspapers a decade ago, in part because of the high bar the Supreme Court has set for determining defamation of a public figure or official.

Denham said Friday that Republican attorneys told him that it was very rare, if not unprecedented, for an incumbent to file a defamation lawsuit over a campaign ad. He said he was motivated to act because “this was personal.”

Politically, the lawsuit that Turlock-based attorney Michael Warda filed electronically in federal court Thursday night in Sacramento sends a definite signal about Denham’s convictions. It also sheds a brighter light on how political ad designers cherry-pick facts to convey arguably misleading impressions.

“Just because I’m a political figure does not give them a right to lie about my record,” Denham said in Friday’s interview.

A Hernandez campaign representative declined to comment Friday.

The two men are wrapped up in an expensive race that’s gotten snippy on both sides. Political parties and outside groups already have spent more than $6.4 million on the race, according to figures compiled by the nonpartisan Center for Responsive Politics.

The money has funded ads such as one sponsored by the Republican group American Action Network that depicts Hernandez in what appears to be a cartoon low-rider car driving across a border into California.