Former Sen. John Warner (R-Va.) is endorsing Sen. Mark WarnerMark WarnerFive takeaways from a busy day of Russia hearings Senate Intel panel issues subpoenas to Flynn businesses Overnight Cybersecurity: Flynn refuses to comply with Senate subpoena | Chaffetz postpones hearing with Comey | Small biz cyber bill would cost M | New worm spotted after 'Wanna Cry' MORE's (D-Va.) reelection, according to The Associated Press, giving the younger Warner a big bipartisan boost for his reelection battle.

The two Warners, who aren't related, are old rivals: John Warner bested Mark Warner by a 5-point margin in the 1996 Senate race, in the latter's first bid for office.

They developed a good working relationship while Mark Warner was Virginia's governor, and the older Warner helped the younger Warner, who succeeded him in the Senate after his retirement in 2008, in his transition into office.

John Warner, one his party's strongest centrist voices, remains a popular figure in Virginia. He's helped candidates of both parties in recent years, most recently endorsing Virginia attorney general candidate Mark Obenshain (R) last year and donating to Georgia Democrat Michelle Nunn's 2014 Senate race.

Mark Warner is also popular across party lines. He's likely to face former Republican National Committee Chairman Ed Gillespie (R), who won Obenshain's endorsement earlier on Monday, in the general election.