The parents of Sen. John Ensign (R-Nev.) — who is the subject of a Senate ethics investigation — in September pitched in to help Majority Leader Harry Reid’s (D-Nev.) 2010 re-election battle.

Michael and Sharon Ensign, who made waves in July after admitting to giving $96,000 to their son’s former mistress and her family, each gave the maximum $4,800 in contributions to Reid’s campaign committee in September, according to the Senator’s recently filed third-quarter fundraising report.

Despite being on opposite sides of the partisan divide, Reid and Ensign have an unofficial non-aggression pact, and Reid has remained silent on his colleague’s admission over the summer that he carried on an affair with Cynthia Hampton, a campaign staffer and the wife of one of Ensign’s former top aides, Doug Hampton.

Watchdog groups have requested investigations into the payments to the Hamptons — which the Ensigns’ lawyer characterized as gifts — and also into accusations leveled by Doug Hampton that Ensign helped him obtain lobbying clients after Hampton learned about the affair, and then helped push those clients’ interests. Earlier this month, Senate Ethics Committee Chairman Barbara Boxer (D-Calif.) acknowledged that her panel has opened an investigation into whether Ensign broke the chamber’s rules.

Michael Ensign is a former casino executive with Mandalay Resort Group, and he and his wife have been major Republican donors throughout the years. Federal contribution records show that they also gave $10,000 apiece to the Nevada Democratic party in 2008, and gave to Rep. Shelley Berkley (D-Nev.) during her 2006 re-election campaign.

The September donations are not the Ensigns’ first to Reid, who has been a friend to gaming interests during his time in the Senate. They also gave $4,000 each to the Democrat for his 2004 election, a race that he won comfortably.

Reid has a big target on his back in 2010, thanks to his growing unpopularity in the state. A number of Republicans are in the running for their party’s nomination. To fend off what is likely to be a tough challenge, Reid has already amassed an $8.7 million war chest, and he last week launched his first television advertisements of the campaign.