The question in the background of Noam Scheiber's exploration of whether Elizabeth Warren could challenge Hillary Clinton in 2016 is whether, come 2015, Democrats will care very much about cracking down harder on Wall Street. An issue-based challenger needs two things to pose a serious threat to a front-runner. One is an issue that differentiates them from the front-runner. The other is for that issue to be foremost in the minds of voters. Scheiber points out that Warren's background isn't just Wall Street reform. It's protecting the middle class. The Two-Income Trap was a seminal book in defining the mounting pressures on working Americans. And Warren has long tried to protect that dimension of her work from being eclipsed by her unexpected turn as a finreg rockstar. But concern for the middle class doesn't, by and large, differentiate her from Clinton. Clinton, like Warren, believes in higher taxes on the rich and universal health care and higher-education costs and universal pre-k and so on. The danger for Clinton is if Warren is able to persuade Democrats that cracking down on Wall Street reform is the key to helping the middle class or -- perhaps more plausibly -- opposing inequality. On a policy level, that's a harder case to make. But on an emotional, who's-on-your-side level, it might work.