It's an old political axiom: Your opponent can't talk if your fist is in his face.

And so it goes in Pennsylvania's Democratic U.S. Senate primary, as Sen. Arlen Specter argues that Rep. Joe Sestak's first TV ad violates Pentagon regulations by using pictures of the former Navy admiral in uniform without a disclaimer.

Those rules allow retired soldiers, sailors, airmen and Marines to discuss their service and display uniformed images of themselves in political communications - as long as they attach a label that says "military images and information do not imply endorsement by DOD [Department of Defense] or service branch."

The complaint comes on the heels of a Specter ad that cast doubt on the end of Sestak's military career and his voting record in Congress. Both attacks hint that Specter strategists are concerned that the incumbent will be vulnerable as Sestak becomes better known.

"If you are a well-known name brand, it's hard to rebrand yourself," Chris Borick, a pollster and political science professor at Muhlenberg College, said Friday. "Joe Sestak has no brand yet, and he has room to grow."

While the omission of a disclaimer might seem minor, the Specter campaign says it is a character issue.

"It's a pattern. He wants to have one set of rules for himself and a different set for everyone else," said Specter campaign manager Chris Nicholas. Earlier in the race, Specter attacked Sestak for paying the majority of his political staff less than minimum wage.

Sestak dismisses the flap over a few words on a TV screen as a trivial distraction.

"He is so desperate to stay in Washington that he's creating phony controversies," Sestak spokesman Jonathon Dworkin said. "We need a senator who will stand behind his record."

Sestak, a two-term House member from Delaware County, is challenging Specter, a five-term incumbent running for reelection for the first time as a Democrat. Sestak began airing an ad Wednesday highlighting his 31-year Navy career, which included stints in the White House and Pentagon and command of a carrier battle group in the Afghanistan war.

Immediately, Specter responded with an attack ad saying Sestak had been dismissed from a top Pentagon job for a "poor command climate" and blasting him for missing 127 House votes. His campaign also challenged the propriety of the military pictures in Sestak's spot.

Specter, known as a ruthless campaigner, has a history of using seemingly small issues to make larger cases against opponents with prosecutorial zeal.

In 2004, for instance, Specter used a radio ad in the GOP primary that accused challenger Pat Toomey of lying when he said his elderly parents were having trouble finding a doctor in Pennsylvania because of medical malpractice costs in the state. The ad said the couple lived in Rhode Island. Toomey said they spent half the year in Pennsylvania.

Toomey wrote to Specter Friday urging him to pull the ad against Sestak, saying, "It is highly regrettable that you have chosen to disparage an honorable man's military service in order to promote your own political career."

Pentagon regulations have long barred use of government video or still pictures in political ads, and active-duty personnel cannot participate in politics in uniform. In 2008, a directive said retired military personnel could use their service in political material as long as they stipulated that the Pentagon was not endorsing them, and as long as the military experience was not the primary focus of the ad.

The Department of Defense general counsel's office has contacted candidates in violation of the policy and asked them to take down or change their ads, said Maj. April Cunningham, a Pentagon spokeswoman. In some cases, Pentagon officials noticed problems, and in others, complaints were referred to the department, she said. She said she did not know how many cases had been flagged.

It's not hard to find candidates ignoring the disclaimer requirement. Democrat Cal Cunningham, a candidate for U.S. Senate in North Carolina, is running an ad that shows footage of him in uniform in Iraq.

In the presidential campaign of 2008, Sen. John S. McCain did not use a disclaimer in one ad that showed him as a prisoner of war in Hanoi, as taken by a French documentary crew. He was not in uniform, however. Another ad that showed McCain in uniform with his fighter jet and being pulled from the wreckage after he was shot down did use the disclaimer. McCain was a stickler for the rule, said a consultant who worked on the campaign.

In 2008, at least three candidates for Congress used images of themselves from military days without disclaimers: Democrats Ashwan Madia of Minnesota, a Marine who served in Iraq; John Boccieri of Ohio, shown as an Army helicopter pilot in Iraq; and Gary Peters of Michigan, shown in his Navy lieutenant commander's uniform.

On Friday, Vice President Biden urged Democrats to reelect Specter, saying he was a key ally of the administration on issues such as health-care and the stimulus.

"This is a man who has as much energy, as much passion, as strong a commitment to ending the injustices that surround us, as anyone I've ever known," Biden said at a rally at the Scranton-Wilkes-Barre airport.

Sestak wrote to Biden asking him to denounce Specter's attack ad, comparing it to the Swift Boat attacks on presidential candidate Sen. John Kerry in 2004. A group of veterans supporting Sestak delivered the letter to the vice president's staff at the airport, but Biden did not address the matter.

John Kennedy, a professor of political science at West Chester University, said there was risk of a backlash against Specter from the ads.

Sestak is "reminding Democrats of all the reasons why they've hated Specter: They got their heads beat in by him for the last 30 years," Kennedy said.

"Why engage Sestak like that?" Kennedy said. "He just gave Sestak a whole narrative to publicize his campaign."

Contact staff writer Thomas Fitzgerald at 215-854-2718 or tfitzgerald@phillynews.com.

Inquirer staff writer Joelle Farrell contributed to this article.