When you think about it, the Democrats didn't really have a message in the 2014 midterms.
Of course, Senator Warren is trying to bring the Democratic Party back to a form of populism not as the "pro-union" party but as the "anti-corporation" party. There's quite a difference there.
And with the GOP conforming to most social norms in the US (or at least not talking as much about social issues), that really isn't much of a booster to the Dems (see: Colorado Senate race).