Senate Minority Leader Harry Reid (D-Nev.) said he was "very disappointed" when Sen. John McCain (R-Ariz) didn't help Senate Democrats pass campaign finance reform. (C-SPAN)

Senate Minority Leader Harry M. Reid (D-Nev.) has labored for years to make the Koch brothers Public Enemy No. 1 when it comes to money in politics.

His disgust for the libertarian industrialists and mega-GOP donors was on full display when he spoke on the Senate floor on Thursday, the sixth anniversary of the Supreme Court's Citizens United decision, which opened up unlimited campaign spending for outside groups.

But in that same speech lamenting the Kochs' role in the demise of democracy, Reid made known his frustrations with another person in the campaign finance world -- a person who is arguably one of its most significant champions: Sen. John McCain (R-Ariz.).

"My friend, the senior senator from Arizona, had an opportunity to help this bad, bad financial system the Supreme Court has put forward, and he didn't step forward," Reid said, according to a transcript. "He decided to take a pass on it. I'm very disappointed. I've never forgotten what he didn't do. He could have done it. One vote. We only needed one vote. We had 59 and needed one more."

In case you can't tell, McCain and Senate Democrats have a complicated relationship on this issue.

To quickly recap what Reid is talking about here, after the Supreme Court's 2010 decision, Senate Democrats tried to pass a bill that would have required outside groups to disclose their big-money donors. As Reid details, Democrats, who had control of the chamber then, needed just one Republican vote to overcome a GOP filibuster. They were counting on McCain -- who sponsored the 2002 McCain-Feingold campaign finance reform act that, in part, prohibited what Citizens United now allows -- to be that 60th vote.

McCain wasn't.

From Democrats' point of view, McCain let them down again in 2012 when they tried to get the campaign finance reform bill through the chamber. Progressive publications such as Mother Jones and the Nation wondered if this time, McCain really would help "save" campaign finance reform. He voted against that bill, too, calling it "a clever attempt at political gamesmanship" that benefited unions more than other groups.

So what's Reid getting at here, bringing all this up nearly four years later? With the caveat that we can't read the senator's mind, we do know that McCain is not a lock to win election to a sixth term, and Democrats would love to see him -- among other potentially vulnerable Republicans -- fall in November so they can take back the chamber. McCain faces Rep. Ann Kirkpatrick (D-Ariz.).

Along those lines, Reid is probably acutely aware of how well 2016 candidates on both sides are doing by bashing billionaires, or at least making clear how unbeholden they are to them. Maybe that wasn't so much the case in 2010 or 2012, but campaign finance reform could be a winning issue for Senate Democrats in 2016.

A final possibility (that's not mutually exclusive from the others): The retiring Reid could simply be fed up with how little progress Congress has been made on the issue. And alongside the Koch brothers, he blames the guy whom Democrats once called an ally on this issue: John McCain.