To Clinton, Democrats are the party of progressives, and so stuff that Democrats routinely do is, by definition, compatible with being progressive.
But though Democrats are certainly the more left-wing of the two parties — the party of labor unions and environment groups and feminist organizations and the civil rights movement — they're not an ideologically left-wing party in the same way that Republicans are an ideological conservative one. Instead, they behave more like a centrist, interest group brokerage party that seeks to mediate between the claims and concerns of left-wing activists groups and those of important members of the business community — especially industries like finance, Hollywood, and tech that are based in liberal coastal states and whose executives generally espouse a progressive outlook on cultural change.
Sanders's core proposition, separate from the details of the political revolution, is that for progressives to win they need to first organize and dominate an ideologically left-wing political party that is counterpoised to the ideological right-wing Republican Party.
[T]here's little reason to believe that actual voters endorse the "lanes" schema that political journalists have embraced. Voters who like Christie's tough-talking persona may be drawn to Trump as the next best thing. Kasich and Trump stand out as the two candidates in the field who are a bit soft on the welfare state. Rubio and Trump are running on similar themes of rescuing the United States from Obama-induced decline. And then of course national polling still shows a healthy ten percent of Republicans backing outsider figures Ben Carson and Carly Fiorina, who may naturally gravitate toward Trump.
1. Party elites don't really matter and endorsements are a terrible predictor of primary outcomes.
2. Retail politics isn't important, especially in New Hampshire.
3. Socialist is a winning affiliation in American politics.
9. Being a member of the political party whose nomination you're running for confers absolutely no advantage in winning said nomination.