During the first Democratic debate, Hillary Clinton was asked which enemies she was most proud of making during her long career in public life. She ticked off a few easy villains — the National Rifle Association, the insurance industry, the Iranians — and then added a three-word kicker that will haunt the rest of her candidacy, and perhaps her presidency: "Probably the Republicans."

On Monday, Joe Biden — perhaps you've heard he's considering a run for president? — gave a speech on climate change to an audience of business leaders. The speech was, by most accounts, uneventful. But at one point, Biden called Republican Congressman Darrel Issa a friend, and took the opportunity to knife Clinton.

"I don't consider Republicans enemies," Biden added slyly. "They're friends."

If Biden does run for president, this is likely to be the core rationale for his campaign. The next Democratic president is going to be facing a Republican House and will need to work with Republicans to get big things done. Hillary Clinton is loathed by Republicans, and she loathes them back — working with people she describes as enemies is not likely to be her strong suit. Moreover, voters are exhausted by bickering in Washington and disappointed that Obama's much-hyped era of unity never came to pass.

But if Obama proved more polarizing than voters hoped, Biden has been a more effective bridge to Republicans than anyone expected. As I wrote in a 2013 piece about the rationale for a Biden candidacy:

A bit over four years ago, Obama was elected on a promise to change Washington. He and his team argued that what politics needed was something new — a post-partisan, post-boomer, post-racial president who could help the country move past old antagonisms and into a more united future. The Obama administration has had many successes, but ratcheting down partisanship isn’t one. The irony, of course, is that the one bright spot in the White House’s dealings with Republicans has been Biden’s old-school, back-slapping, Senate-steeped, Washington-lifer approach.

Biden is the guy who negotiated the fiscal cliff deal with Mitch McConnell. He was also a major part of the 2011 deal to lift the debt ceiling and the 2010 deal that extended the Bush tax cuts in return for fresh stimulus.

There's a reason for that — Republicans just plain like him. "We probably disagree more than we agree," former House Majority Leader Eric Cantor said of Biden, "but from a human and relationship standpoint, the guy’s awesome."

If Biden does run for the Democratic nomination, this is how he'll do it — by arguing that Democrats aren't going to get anything done amidst the partisan warfare and endless investigations of a Hillary Clinton presidency. The Benghazi hearings and the email scandal have been a reminder of what a Clinton White House might look and feel like, and Biden's going to be betting that it isn't exactly leaving Democrats thrilled, particularly given their near-certain disadvantage in the House.

Is it a fair argument? Partially. Biden's effectiveness has likely been overstated by his position. Part of the reason Republicans made deals with him was that it was a safe way to make a deal with the White House — it let them make the deal they wanted to make while showing that President Obama was too extreme and partisan to work with. As president, Biden would be more polarizing, and Republicans would be less cooperative.

But relationships do matter in Congress, and there's little doubt that Biden has among the deepest congressional ties of anyone in Washington. The result is that Biden's backslapping bonhomie with congressional Republicans lets him draw a contrast with both Obama's cerebral style and Clinton's more Manichean worldview, and that may be a contrast that voters find appealing.