In many states and in the federal government, legislators are immune from arrest while serving. The purpose is to prevent political opponents from interfering with them doing their duty through trumped-up charges. Unfortunately, if there is a privilege to be abused, there’s always a Republican available to abuse it.

The majority leader of the Arizona State Senate scuffled with his girlfriend during an argument on the side of the road late one night recently. He hit her and she hit him, according to the police, but the two suffered dramatically different fates.

The majority leader, Scott Bundgaard, told Phoenix police officers that he was a state senator, and he cited a provision of the Arizona Constitution that gives lawmakers limited immunity from arrest, the police said. Police Department lawyers were consulted, and they ordered that Mr. Bundgaard be uncuffed and released.

Aubry Ballard, Mr. Bundgaard’s girlfriend of about eight months, on the other hand, was arrested for domestic violence and spent the night in jail.

Just how protected lawmakers should be from prosecution is an issue that many states grapple with, said Steven F. Huefner, a law professor at Ohio State University who studies the issue.

He said the privilege, which is included in the United States Constitution and in many state constitutions, was designed to protect lawmakers from civil matters that would interfere with their legislative duties. “The legislative privilege should not become a get-out-of-jail-free card or escape-from-ever-being-put-in-jail card for state legislators,” he said during a presentation on the issue during the National Conference of State Legislators Summit last year.

The special treatment that Mr. Bundgaard received, and the domestic violence accusations against him, have drawn considerable criticism here, with some of the senator’s colleagues and women’s groups calling on him to resign, or at least step down from the Senate leadership.

Intent on holding onto his job, Mr. Bundgaard, 43, denied that he invoked legislative immunity after the police responded to his roadside brawl with Ms. Ballard on Feb. 25. He said that Ms. Ballard, 34, hit him after accusing him of dancing the rumba too closely with another woman in a local charity version of “Dancing With the Stars.” He said that he did not hit Ms. Ballard at all and that he passed a polygraph.

Sgt. Tommy Thompson, a police spokesman, said in an interview that the senator specifically invoked Article 4 of the State Constitution , which says lawmakers are “privileged from arrest in all cases except treason, felony and breach of the peace, and they shall not be subject to any civil process during the session of the Legislature, nor for 15 days before the commencement of each session.”

Ms. Ballard has accused her ex-boyfriend — both of them say the relationship is over — of hitting her first as they drove in his gold Mercedes on State Highway 51 north of downtown Phoenix. They have accused each other of throwing personal items out of the window of the moving car, which Mr. Bundgaard eventually pulled over near the median.

After he hired a public relations consultant to present his version of events, Ms. Ballard went on local television [Faux delinked] to give her side of the story. “The officer came over, the sergeant, and said, ‘Look, I hate to do this to you, it’s not fair, but I’m going to have to take you off to jail. He’s been granted immunity; he’s a senator,’ ” she said.

Police departments around the country treat legislators’ privileges in various ways.

In 1999, Senator Robert C. Byrd of West Virginia pulled out a copy of the United States Constitution after a traffic accident and pointed out the section that stated that members of Congress “shall in all cases, except treason, felony and breach of the peace, be privileged from arrest.” He later backed down and had an aide ask the police in Fairfax County, Va., to issue him a citation.

In Arizona, State Representative Mark DeSimone was cited for misdemeanor assault for hitting his wife in the face in 2008, The Arizona Republic reported. The charge was dropped after he resigned and agreed to undergo counseling.

At other times, the paper reported, lawmakers have faced no penalty. That was the case in 1988 when the police released State Senator Jan Brewer, who is now Arizona’s governor, after discovering she was a lawmaker. She had been involved in a car crash and had stated that she had been drinking.

A group of Democratic lawmakers have demanded that Mr. Bundgaard, the Arizona Senate’s No. 2 Republican, resign. The Senate president, Russell Pearce, has stood by his Republican colleague, saying that he considered Mr. Bundgaard to be the “victim” in the case. At a closed-door caucus meeting on Tuesday, Republicans declined to remove Mr. Bundgaard from his leadership position… [emphasis added]