Dems can allow themselves to do bare minimum because they know that you don't have other choice, because not voting or voting 3rd party is a terrible idea. They can occasionally do more than bare minimum, but at its core two-party system basically holds country hostage
Not to mention that many of them really want what the Republicans are trying to pass, especially when it comes to economic issues, so they can just deliberately not put up much of a fight to ensure Republicans get what they want and so that it looks like they're progressive from the outside.
The "third parties" that run as part of the two parties are smarter. Think Bernie or the Tea Party. Then you can vote for them in the primaries without being used as a tool by the other party.