You make the same mistake that most make in assuming Democrats actually want to hold a majority large enough to put them in the driver's seat. They don't and they are notorious for snatching defeat from the jaws of victory to make certain that any such majority is temporary.

Democrats abandoned the working class decades ago and now serve the same donor class Republicans are beholding to. They are most comfortable in the role of "resistance" where they can throw stones at their competitor Republicans without actually threatening their donors. The only question is who benefits from the corruption and subjugation of Americans, not who's policies are best for them.