According the Neustadt the one thing a good US President needs to be able to do is to persuade people. The lack of formal powers means recourse to the dark arts of persuasion (of America or Congress) are necessary if a President is to acheive anything.
This piece in the New Yorker challenges this assumption, and suggests there is little quantifiable evidence of Presidential persuasion.
In addition the article puts forward an interesting view on Congress
“We’ve effectively lost our Congress and gained a parliament.” He adds, “At least a Prime Minister is empowered to get things done,” but “we have the extreme polarization of a parliament, with party-line voting, without the empowered Prime Minister.”
This comparisson with the UK system is an excellent piece of evaluation for a question on the power of Congress.