Former Sen. John Warner, R-Va., reached across the aisle Monday to endorse incumbent Sen. Mark Warner, D-Va., over his GOP opponent in this year's U.S. Senate contest.

This marks the first time that John Warner has publicly endorsed a Democrat in Virginia, although he did decline to endorse Mark Warner's GOP opponent in 2008.

Mark Warner's best-known opponent, Ed Gillespie, is a former chairman of the national Republican Party and senior adviser to Mitt Romney's 2012 presidential campaign.

Gillespie officially announced his candidacy for the seat earlier this month.

John Warner praised Mark Warner's bipartisan efforts in his endorsement for a second term.

"The Senate works best when there's collaborative effort between the two parties," Warner said, according to the Associated Press. "There are times you must, I think, recognize that certain individuals are superior in their talents and in what they have done and can potentially do for your state."

Gillespie responded in a statement, "I respect Senator John Warner for his service to our country and our commonwealth, and respect his right to support the candidate of his choice in this race."

Although the Warners are not related, the two have a longstanding history. In 1996, Mark Warner tried to defeat then-Sen. John Warner's bid for a third term, but lost by 5 percent of the vote. Mark Warner then ran for governor and was elected in 2001, and in 2008 he won John Warner's seat after he announced plans to end his 30-year Senate tenure.