Earlier this week, Sen. Elizabeth Warren stated unequivocally that she would not run for president in 2016: "I am not running for president and I plan to serve out my term." That's it, then, right? No how, no way.

On the other hand, when Tim Russert asked then Senator-elect Obama in November 2004 if he would serve out his full Senate term, Obama replied: "Absolutely." But hold on a minute, that was four years before the 2008 election. Well, in January 2006—right about where we are now in terms of the 2016 cycle—Russert asked Obama if he would run for president or vice president in 2008. "I will not," he replied. Nine months later gave birth to "I have thought about it over the last several months." Four months after that, Barack Obama stood in front of a cheering crowd in Springfield, Illinois, and announced his candidacy for the White House. So much for filling out his term in the Senate.

I bring this up not to criticize President Obama or to question the sincerity of his early denials. Nor do I bring this up as evidence that Sen. Warren is going to run for president, or even that she is considering doing so. I take her at her word when she says she is not considering it. My point is this: things change. Remember that.

Why are so many Democrats excited about the prospect of an Elizabeth Warren candidacy and presidency? Because our party, like all parties at all times, has to decide which values it will champion. We must remember that the best chance we have to move our party in the direction we believe it must go is to nominate strong progressives. It's all about the primaries.

Among the top echelon of potential 2016 presidential nominees, I don't see a more effective fighter for progressive values—in particular the fight for the 99 percent against the concentration of power and wealth among the 1 percent—than Elizabeth Warren. So, even though there are other strong candidates for our party's nomination, people I would have no trouble backing in a general election campaign, I'm going to keep talking about Elizabeth Warren.

One thing I want to address is the idea I've run across that progressives shouldn't want Warren to run for president because somehow she can do more in the Senate. This may be folks trying to make lemonade out of lemons, and there's nothing wrong with that. In any case, I have two responses. First, the idea that any senator can have a greater influence than a president on shaping the direction of our country is simply incorrect. It's impossible given the power a president holds in our system of government versus that of one senator. Second, the act of running for president in no way takes Elizabeth Warren out of the Senate. She's not up for re-election in Massachusetts until 2018, and there is no need for her to resign in order to run. Can she be a highly effective senator in the mold of Ted Kennedy if she remains in that august body for a decade or two? Of course. But let's not kid ourselves about where she could be most effective.

Follow me beyond the fold for more discussion of now and then, and Obama and Warren.