To put it bluntly, the idea was Democrats could finally run as the "good" conservatives they always wanted to be and the left-leaning, working class, and marginalized members of their coalition would have no choice but to support them against someone like Trump.
Chuck Schumer put it thusly: "For every blue-collar Democrat we lose in western Pennsylvania, we will pick up two moderate Republicans in the suburbs in Philadelphia, and you can repeat that in Ohio and Illinois and Wisconsin."