David Koch in an August 2013 A.P. file photo. (Associated Press)

DETROIT, MI -- A group of high-profile conservatives plans to file an amicus brief with the U.S. Supreme in support of gay marriage ban challenges out of Michigan, Ohio, Kentucky and Tennessee.

Among the brief's signatories will be David Koch, a major donor to Republican and conservative groups, according to the Washington Free Beacon.

Retired Army Gen. Stanley McChrystal, former Republican National Committee Chairman Ken Mehlman, former White House chief of staff Ken Duberstein and conservative pundits S.E. Cupp and Alex Castellanos are also expected to sign the document, according to the Washington Post.

The news may come as a surprise to some liberal gay rights supporters, but it's not exactly a new development.

Koch expressed his support for gay marriage in a 2012 Politico interview.

And a group of 25 Republicans, most of them former members of the Michigan legislature, argued in favor of allowing gay marriage, even calling the position a conservative one, in an amicus brief filed last year when Michigan's same-sex marriage ban was before a federal appeals court. (Google, Starbucks, 25 Republicans and 16 states make arguments in Michigan gay marriage case)

The Supreme Court's is expected to hear arguments on the issue in April and issue a decision by June.

Michigan's role in the case stems from a lawsuit filed by a Hazel Park lesbian couple who sued the state in 2012 because they can't jointly adopt their children without a legal marriage in Michigan.

The case initially challenged Michigan adoption laws, but at the suggestion of a federal judge, Jayne Rowse and April DeBoer amended their complaint to challenge the voter-approved gay marriage ban.

After a long wait for their day in court, a nine-day federal court trial, a ruling in their favor, a hectic 24 hours in which some 300 Michigan gay couples were married and an appeals court ruling overturning the first, the U.S. Supreme Court on Jan. 16 agreed to review their case.

Lawyers for the Michigan couple filed their Supreme Court brief Friday, arguing that preventing same-sex parents from marrying leaves kids vulnerable to economic and psychological harm. (Michigan couple's Supreme Court brief in gay marriage case focuses on 'injury to children')

A response brief from lawyers for the state is due by March 27.