Kaine had run an ad promising to work with a president of either party, noting previous collaborations with President Bush during his tenure as governor. He said he had disagreed with Obama on certain issues, but ultimately, the two just didn’t differ on all that much. “He carried that approach with him to the Senate,” Elleithee said, “and I think he got a lot of street cred for that—both with the White House and with people back home.”

A year and a half after arriving in the Senate, however, Kaine did break with the man he once served in a much more vocal way: He called on Obama to seek congressional authorization for the use of military force against ISIS and began pushing back against the administration’s claims that it had all the authority it needed to launch airstrikes in Iraq. “President Obama said in May 2013 that he would work with Congress to update the 2001 [Authorization for the Use of Military Force],” Kaine wrote in an op-ed for The Washington Post.

It is June 2014, and there has been no progress. The White House should submit to Congress a new draft authorization to deal with today’s threats. Now is clearly the time for this debate. I believe the president must come to Congress for authority to initiate any U.S. military action in Iraq.

It would be another six months before Obama would actually ask Congress for a new AUMF, a request that came long after he began dropping bombs in Iraq and Syria. Kaine has continued pushing for Congress to act, but more than two years after his initial demand, neither the House nor the Senate has been able to agree on language that both authorizes and constrains the war against ISIS.

An aide to Kaine, speaking on the condition of anonymity because he is not an authorized spokesman for the senator, said he became interested in the War Powers Resolution and Congress’s role in declaring war after studying a 2007 report issued while he was governor by a bipartisan commission at the University of Virginia’s Miller Center. The recommendations became the basis for legislation he introduced with Senator John McCain in 2013 to revise the 1973 law.

Those close to Kaine said his outspoken advocacy for a resolution authorizing the war against ISIS flowed from a genuine belief in Congress’s responsibility to weigh in, both from a Constitutional and a moral standpoint. But they don’t deny that it also helped him politically on two key fronts.

When Obama considered tapping him for the vice presidency in 2008, Kaine had been serving as Virginia’s governor for just two and a half years. He had no foreign-policy experience, and had been in statewide office less than a year longer than Sarah Palin. Kaine scored seats on both the Foreign Relations and Armed Services Committees in the Senate, but taking a high-profile stand on the AUMF was clearly an opportunity for him to bolster his credentials on the military and foreign affairs.