PARIS (Reuters) - Lawyers for former IMF chief Dominique Strauss-Kahn will ask a court on Wednesday to close a sexual misconduct investigation that has plagued their client, once the runaway favorite to become president of France, a judicial source said.

The hearing is due to take place behind closed doors in the northern city of Lille, where investigators have spent months trying to verify allegations that Strauss-Kahn took part in sex parties with prostitutes and did so in ways that broke the law.

His lawyers, who argue that he is being witch-hunted because of his self-confessed penchant for libertine sex, are appealing over the heads of the judicial investigation team to have the allegations against the 63-year-old dropped.

A lawyer for Strauss-Kahn was not immediately available for comment.

Strauss-Kahn was about to enter the French presidential race when police arrested him in New York in May 2011 on the basis of accusations by hotel maid Nafissatou Diallo that he tried to rape her - charges that U.S. prosecutors later dropped.

His hopes of rapid rehabilitation have foundered ever since his return to his native France, where his wealthy art heiress wife, Anne Sinclair, has shunned the role of supportive spouse to revive her career as a journalist.

Police inquiries into sex parties and prostitution in Lille snowballed earlier this year as prosecutors sought to establish whether Strauss-Kahn actively promoted prostitution through his involvement in organized sex parties, and in May broadened their inquiry to include allegations of group rape.

Having sex with prostitutes is not in itself illegal in France.

(Reporting by Pierre Savary; Writing by Brian Love; Editing by Mark Heinrich)