As a Congressman, it's hard to fight against upper-level pressure (particularly when the President is for open borders).
As a President, it's far easier to get things done.
Bwahahahaha. gasp. Bwahahahaha. Tell that to President Bush.
Heck, tell that to the last, oh, I don't know 43 Presidents. It's hard to get things done no matter which branch of the government you belong to. Intentionally so. LOL.