That's the point of the firebrand President. We have not had a President since FDR who knows how to stand up to special interests and even their own party when necessary.
You mean on principal? Those in Congress with principals are on our side. Those without principals can be convinced it's in their best interest. Don't fall for the facade that anything that happens in Congress is ideologically driven.
Their best interest is also whatever helps keep them in office. Terrible representatives are typically only secure in their offices if they have the protection of the party. The President has as much power as leader of their party as they get from their actual office. FDR used that to great effect.
The point is that it's a self-imposed handicap. If a party has 50%+ in both houses and the presidency, they have the ability to pass whatever they like and choose not to use it.