This is what a 57-41 GOP popular vote win looks like based on a uniform swing from 2012, with a few adjustments in the margins due to favorite-son effects. This is Sanders' floor imo. Also, note that a lot of the Dem states that flip to Republican are very close in this map.
EDIT: Forgot to report that that's a 418-120 GOP win.
There is no way Nevada, New Mexico, and Florida are close, especially with Jeb Bush and Marco Rubio. In Florida, they have home advantage, but in all of them, they will do very well with hispanic voters. Bernie Sanders sought to defeat immigration reform in 2007, arguing it would lower wages for American workers. He isn't a big fan of a guest worker program.
Also, Pennsylvania and Ohio would not be that close, you can't be too far left or right to win in either state, and Sanders is very far left.
New Hampshire and Maine go for Democrats more often than Republicans, but the GOP has had some success in both over the last 23 years it has gone Democrat for President.
Virginia only votes Democrat now because of federal employees moving there. I think those federal employees, despite their somewhat progressive values, know that Bernie Sanders is not capable of being President.
While President Obama won comfortably in Oregon twice, President Bush was within 4 points of winning there in 2004, in a blue state.
Since 2008, for statewide offices, Wisconsin went Democratic twice (2008 and 2012 for President Obama, granted narrowly in 2012 for a blue state) but also Republican three times (2010, 2011, and 2014 with Ron Johnson & Scott Walker). With Wisconsin's Paul Ryan on the ticket in 2012, Mitt Romney reduced President Obama's margin of victory in Wisconsin from 14 points to 7 points over 2008, and President Bush lost Wisconsin by less than 1 percent in 2004.
Iowa is a state that has been good to both parties, but statewide, no one that far to the left, with the exception of President Obama who is still not as left as Bernie Sanders, has won that state.
I live in New Jersey. We have elected liberal Democrats, but not those who claimed to be socialists. Additionally, to be quite blunt, some Democrats in suburban communities would not vote for someone like Bernie Sanders who comes across as a nut, and we have a considerable Hispanic population.
Connecticut has a lot of wealthy people who are Democrats, but not socialists.