Hillary Clinton's attack on her primary opponent, Sen. Bernie Sanders (I-Vt.), basically comes down to two divergent arguments:

Sanders is either 1) too willing to work with the other side, having voted with Republicans in the past on issues like gun control and against releasing the money that ended up funding the auto industry bailout. Or 2) he's not flexible enough, promising voters universal health care and free tuition despite the fact these proposals have little chance of making it through a Republican -- or any -- Congress.

Now, we have data to back up one of those claims. According to a nonpartisan ranking out Monday from the the Lugar Center and Georgetown University’s McCourt School of Public Policy, Sanders was ranked the least-bipartisan senator in 2015, a notable feat in a year the Lugar Center says is among one of Congress's least-bipartisan in decades. Sanders actually beat out his fellow presidential candidate, Sen. Ted Cruz (R-Tex.), who came in 97th out of 98 senators. (The Senate's Republican and Democratic leaders, whose jobs dictate they are necessarily partisan, weren't included.)

The index measured how many times a senator sponsored or co-sponsored a bill with the other side. The rankings mean Sanders almost exclusively signs onto bills with Democrats, and Cruz with Republicans. (Sanders is technically an independent, but for this purpose, he was categorized as a Democrat, the party with which he caucuses.) The other senator still in the race for president, Sen. Marco Rubio (R-Fla.), ranked 28th most bipartisan.

It's probably not a coincidence that two of the presidential race's most anti-establishment candidates rank as the Senate's most partisan in 2015. There's evidence that both their supporters are deeply unhappy with the status quo in Washington, and both men have tried to fashion a campaign around it. Even though Sanders has been in Congress for two and a half decades, he has crafted a reputation as someone who thinks and works outside the constructs of traditional Washington. In nearly every campaign speech, he champions "a political revolution" to make things right again. Cruz, meanwhile, has staked his image on upsetting official Washington. He has joked on the campaign trail about needing to test his food in the Senate cafeteria for poison.

But Sanders doesn't necessarily cherish or even campaign on his partisan record the way Cruz does. He likes to talk to voters about how he's the most progressive member in the Senate, but he also talks up his efforts to work with Republicans, including Sen. John McCain (R-Ariz.) on veterans issues.

It's likely that running for president from Clinton's left has probably moved Sanders to the left in the Senate as well, contributing to his 98th-out-of-98 ranking in 2015. But the Lugar Center has some evidence Sanders has been a fairly partisan senator for much of his time in the Senate. In December, the center released its bipartisan rankings for 227 senators over the past 10 Congresses, from 1993 to 2014. Sanders was the 11th most-partisan senator, while Cruz was fourth (although the rankings only measured Cruz's first year in office).

Still, the next time Clinton tries to attack Sanders for siding with Republicans in Congress, he can back up his rebuttal with this data. But the next time she accuses him of being too partisan, well ... Sanders will have to decide whether he wants to go down that road or not.