Bernie Sanders on Wednesday attacked Hillary Clinton s political self-identification as a progressive. The Vermont senator posted a series of tweets defining for his supporters what it means to be a real progressive.You can be a moderate. You can be a progressive. But you cannot be a moderate and a progressive.  Bernie Sanders (@BernieSanders) February 3, 2016On top of defining the word  establishment  as a pejorative term, Sanders is now engaged in a game of ideological purity similar to the ongoing battle on the Republican side about what it really means to be a conservative.Contrary to Sanders  view of progressivism, it s absolutely possible to be a progressive on some issues and a moderate on others. Likewise, it s entirely possible to be a progressive while governing as a moderate   be it a moderate across the board or just occasionally depending on the issue. For example, what about the gun control issue? his campaign manager says he is  very moderate  on the issue.  https://t.co/5LX5Pkhuji  Alex Seitz-Wald (@aseitzwald) February 3, 2016So, Bernie s own adviser defined him as  moderate  on guns. How would Sanders score on his own ideological purity test?But, really, what does it even mean to be a progressive? Is there an agreed-upon set of qualifications for the label? Is Sanders talking about being a far-left liberal? A progressive reformer in the classical Teddy Roosevelt sense? What is it? Sanders elaborated via a series of additional tweets in an attempt to define what he means.Most progressives that I know don t raise millions of dollars from Wall Street.  Bernie Sanders (@BernieSanders) February 3, 2016Most progressives I know are firm from day 1 in opposition to the Trans-Pacific Partnership. They didn t have to think about it a whole lot.  Bernie Sanders (@BernieSanders) February 3, 2016Most progressives that I know were opposed to the Keystone pipeline from day one. Honestly, it wasn t that complicated.  Bernie Sanders (@BernieSanders) February 3, 2016Most progressives I know were against the war in Iraq. One of the worst foreign policy blunders in the history of the United States.  Bernie Sanders (@BernieSanders) February 3, 2016Fair points, but why do they have to be couched in the context of a purity test?It appears as if Sanders is taking on both the Clinton machine, the Democratic Party establishment and anyone who doesn t qualify as  progressive  in his view   ejecting them from his exclusively progressive club. It s difficult to see how this helps to build a unified, big-tent coalition necessary to not only win the nomination but, more importantly, the general election.One last thing. It s entirely possible that Clinton believes she s a progressive. The term is fluid enough to apply, even though she likely adopted the term in an attempt to win over voters to her left. But wasn t part of the point of a Sanders candidacy to influence the so-called moderate Democrats to do exactly what Clinton is doing   to be influenced by the strength of the Sanders message and to be pulled leftward? If so, he s clearly been successful at doing so. If he d rather not pull center-left Dems leftward, then he should say so. But he can t have it both ways. Featured image via video screen grab.