"Are you the most liberal president in U.S.history?" That's the question that Fox News Channel's Bill O'Reilly, never one to mince words, put to President Obama during an interview over the weekend. Obama's answer? "Probably not." He then expanded a bit, adding: "The truth of the matter is, is that when you look at some of my policies, um, in a lot of ways, Richard Nixon was more — more liberal than I was. Started the EPA. You know, uh, you know, started, uh, uh, a whole lot of the regulatory state that, uh, has helped make our air and water clean."

The back and forth between O'Reilly and Obama -- and reading the transcript is PAINFUL due to the number of interruptions -- raises one of the lingering questions about Obama's presidency: How liberal is he, really?

It's a good question. Many Republicans insist he is very liberal -- using the Affordable Care Act, which expands the government's role in providing health coverage to Americans, as their main talking point. Louisiana Gov. Bobby Jindal has called Obama the "the most liberal, most incompetent president in the White House since Jimmy Carter." And, Obama still wears the legacy of being named the most liberal senator in National Journal's 2007 vote ratings.

But, what do the numbers say?

That's a more difficult question to answer than you might think. Why? Because, unlike senators and House members, presidents don't vote on legislation, so calculating their relative liberalism or conservatism is more of a guessing game. A group of a half-dozen political scientists has come up with something called DW-Nominate, a ranking system for the relative liberalism/conservatism of members of Congress, that they have extrapolated to presidents using publicly stated support or opposition for legislation.

The group writes:

We find that President Obama is the most ideologically moderate Democratic president in the post-war period, with a first dimension DW-NOMINATE Common Space score of -0.329. President Lyndon Johnson, the second-most moderate Democratic president in this period, has a score of -0.345. President Obama’s ideological position is estimated from his “votes” (statements of support or opposition) on 282 congressional roll call votes. This amount is somewhat low; for example, President George W. Bush “voted” 453 times during his last term in office. However, it is adequate to recover his latent ideological score.

Judging from the DW-Nominate scores, Obama's closest analog, ideologically speaking, is Lyndon Johnson. The rankings also suggest that both Jimmy Carter and Bill Clinton were more liberal than Obama.

And that finding is where Republicans will take issue with the numbers, noting that Bill Clinton was a self-professed centrist who pushed his party to the middle on things like welfare reform. And, the DW-Nominate scores have limitations -- particularly when it comes to presidential rankings because (a) they are inferring what a president's position is without having an actual vote to point to and (b) it's actually really hard to effectively judge political ideology over time.

In a post on the Monkey Cage Blog that ran in May 2011, Georgetown (Hoya Saxa!) political scientist Michael Bailey explains:

In the 1960s we know politicians moved left. At the start of the decade, politicians debated fairly mild civil rights reforms in the face of Jim Crow laws; by the end of the decade, politicians debated how aggressively students should be bused to promote desegregation. But if you look at the NOMINATE scores...you see nothing of this. You see something like this figure: racists from the 1960s like Senators Eastland and Ellender are indistinguishable from the modern moderate Democrats like Fritz Hollings. Eastland openly argued for white superiority on the Senate floor; Hollings voted to override Bush's veto of the 1991 Civil Rights Act. There is huge movement but you see none of it in NOMINATE scores.... If we really want to talk about ideological change, we need to keep track of policies and where politicians move on these policies.

Simply put: Using a single measure to compare politicians over time is extremely problematic due to the decided lack of context that goes with those numbers. It's the same reason that using batting average, for example, to compare the relative greatness of baseball players is a difficult thing to do. Or home runs. (Thanks a lot, steroid era!)

Given the difficulty of measuring the relative ideologies of presidents past and current, the fight over Obama's liberalism devolves into a perception battle.

Republicans, who hate the health-care law as much as any piece of legislation offered by any Democratic president, will point to it as a sign of Obama's deeply held belief that government is the solution to most problems. Here's O'Reilly summing up that sentiment: "I think that you are much more friendly to a nanny state than I am. I’m more of a self-reliance guy; you’re more of a big-government-will-solve-your-problems guy."

Democrats will note that the health-care law was less than liberals wanted (single payer, anyone?) and that on a number of issues -- Gitmo, Wall Street -- he has chosen a far more centrist course than they would like. Here's David Sirota writing about Obama in Salon in 2011: "Obama is not a flaccid Jimmy Carter, as some of his critics insist. He is instead a Franklin Delano Roosevelt — but a bizarro FDR. He has mustered the legislative strength of his New Deal predecessor — but he has channeled that strength into propping up the very forces of 'organized money' that FDR once challenged."

What does it all mean? Obama's liberalism (or not) is in the eye of the beholder unless and until we can come up with a better way to measure the ideology of our presidents. (Nate Silver, we are looking at you.)