A moderate third-party presidential candidate would be a terribly stupid idea and it seems almost beneath E.J. Dionne to waste his time responding to it. The point Dionne fails to make, however, is that President Obama is already a moderate candidate. In fact, any president who is primarily focused on passing legislation (as opposed to starting foreign wars and gutting regulation) is going to be a moderate. This is a poorly understood fact.

To see why this is the case, we can simply look at the U.S. Senate during Obama’s first two years in office. The president’s party had nominal control of between 56 and 60 seats (depending on the time period and health of the senators). Because the Senate now requires 60 votes to pass anything remotely contentious, the president couldn’t pass anything that didn’t have the support of every single Democrat and (for most of those two years) at least a small handful of Republicans. What this meant was that the most conservative (or moderate or centrist) Democrats had effective veto power over bills and amendments to bills. In most cases, the most progressive (or moderate or centrist) Republicans also had effective veto power. In other words, if Ben Nelson, Joe Lieberman, Susan Collins, and Olympia Snowe didn’t like a provision, it wasn’t going to be included in the bill. And if they didn’t like the bill, it wasn’t going to pass.

The president’s ability to move this centrist coalition to the left was limited. One limitation was created by Mitch McConnell’s strategy of total obstruction. He put enormous pressure on the centrist Republicans not to play ball. And that took away the cover that centrist Democrats like to have when they vote with the party on contentious issues. If Ben Nelson can’t find a couple of Republicans supporting a bill, he pretty much wets his pants. At times, Joe Lieberman appeared to be changing his positions for no other reason than to anger liberals. With dynamics like that, it’s kind of hard to push through liberal legislation.

McConnell’s total obstruction strategy forced Obama farther to the middle, but to a large degree all presidents are forced to work with the middle. Even FDR and LBJ’s huge supermajorities were a little misleading, as they were based on the Democrats’ strength in the conservative South. But those majorities were sufficiently large to create significant liberal legislation.

Ask yourself how the Dodd-Frank bill might have turned out if there had been 70 or 80 Democratic senators. How would the health care bill have turned out under those circumstances? What would we be doing about housing in that alternate reality? Would Gitmo be closed?

Presidential candidates make a lot of promises, but their ability to keep those promises is based on their ability to convince 60% of the U.S. Senate to go along with their agenda. And that guarantees that most presidents will pursue fairly centrist legislative agendas, or they will fail.

George W. Bush didn’t sign a whole lot of legislation. He got his tax cuts (under reconciliation rules, with Dick Cheney casting the tie-breaking vote both times) and his education, Medicare Part D, and Bankruptcy Reform bills, and he was pretty much done. When he tried to push something radical (privatizing Social Security), he got absolutely nowhere.

It’s true that a president can have some far right or far left personal beliefs, but they can’t translate the beliefs into action in the legislative field. If you want really progressive or really conservative outcomes, you need to win a very large supermajority in the Senate and have a president willing to take advantage of that supermajority. Without that kind of power in the Senate, all presidents will produce fairly centrist legislative records.

It still matters where the president stands. It’s especially important when the president’s party loses total control of Congress, because a president might be tempted to start signing the opposing party’s bills and try to take credit for them.

It’s one thing to make tough compromises that water down what you want to do. It’s another to embrace the other party’s agenda and then call it your own.