The complaint filed in federal court in South Carolina late Tuesday seeks a restraining order to prevent Attorney General Henry McMaster from filing any such charges. It also contends that Mr. McMaster's warnings violate the U.S. Commerce Clause.
The suit also names the state's 16 solicitors, whom the suit says are Mr. McMaster's agents operating throughout the state "who either may be compelled to act under his direction or may be directly or indirectly influenced to follow-through on his treats to prosecute craigslist and its management."
San Francisco-based Craigslist has come under closer scrutiny for its "erotic services" category after a Boston-area man was accused of killing a masseuse he met through the Web site. The company has promised to eliminate the category and replace it with a new "adult services" section, where ads will be screened before they are posted.
Last year Mr. McMaster and attorneys general from dozens of other states entered into an agreement with Craigslist to improve the site's safeguards. In that document, Craigslist agreed to several defenses, including allowing users to flag pornographic postings they believe violate the Web site's rules.
But Mr. McMaster, a Republican who plans to run for South Carolina governor in 2010, has said that he doesn't think Craigslist has done enough to keep those images, as well as ads for prostitution, off its Web site.
Earlier this month he asked Mr. Buckmaster to take down ads related to prostitution and pornography, giving Craigslist 10 days to comply or face possible charges. When that deadline passed, Mr. McMaster said he would prosecute Craigslist executives for aiding and abetting prostitution if an ad on the Web site leads to a prostitution case in South Carolina.
Mr. McMaster has said his office is investigating, but no charges have been brought.
On his blog, Mr. Buckmaster says complying with Mr. McMaster's demands would require shutting down Craigslist in South Carolina.
In a news release today, Mr. McMaster said the lawsuit shows that Craigslist is taking his investigation seriously and called it "a victory for law enforcement and for the people of South Carolina."
"We trust they will now adhere to the higher standards they have promised," Mr. McMaster said. "This office and the law enforcement agencies of South Carolina will continue to monitor the site to make certain that our laws are respected."
A spokeswoman for Craigslist would not comment on the lawsuit.
Earlier today New York Attorney General Andrew Cuomo's office announced seven people had been indicted on charges of running a 24-hour prostitution ring on Craigslist. Mr. Cuomo's office says the group operated Room Service Entertainment in the "erotic services" section of Craigslist from June 2007 through December 2008. Investigators say the ads included explicit or seminude photographs and a phone number to arrange "dates."