Trump can't fire congressmen. Trump can campaign *against* them when they run for re-election if he wants, but has zero say in congressional leadership or even committee chairs. The voters have *everything* to do with whether congress keeps their jobs. Looking back as far as I can see, it's the *voters* who consistently, like clock-work, re-elect these deplorable chronic capitulators election after election after election (see McConnell, Graham, McCain, etc).
Quite the contrary, the only leverage trump will have against congress may be a voter mandate, which congress can ignore with impunity -- unless they somehow, suddenly become the least bit concerned about their approval ratings. Trump will have to find something congress wants badly enough that he can trade in exchange for the things he wants badly, like his signature illegal immigration policy.
The gazillion dollar question that no one can answer is: What further infringement on the precious little that remains of our liberties is trump willing to trade to a recalcitrant congress in order to advance his agenda? (Yeah, it's a rhetorical question - we can only guess).
As head of the party Trump controls to whom the money flows. That has a big effect on Congress. Congress was neutered by Obama. Trump can easily do the same.
No, Trump cannot fire Congressmen, but he can fire the entire leadership of the party & replace them with his own people. He can fire all those friends, family, & cronies of the Congressmen in the government & in the party.
If Trump wins the Pub nomination, many of the Pub Congress critters up for election are gonna want his endorsement & the votes of his supporters. Thousands of political appointments in government are to be doled out by the new President. The nominee & eventual President carries enormous weight with the voters, & Obama has repeatedly shown a President can get his way despite a hostile Congress.
Trump doesn’t need Congress to enforce the existing immigration laws, which Presidents have ignored for decades. These laws have always been sufficient to stop illegal immigrants & send them back home, if they are enforced. Border agents routinely complain they are not allowed to do their jobs by their politically appointed superiors. If he chooses to withhold funds from sanctuary cities, who can force him to release the funds? No one. Congress proposes but the President disposes, or not.
As for trading away our liberties, the GOP leadership seems in lockstep with the Democrats in expanding government tyranny. While Trump is a bit of a question mark, it is a sure thing that if Rubio, Kasich, or Cruz becomes President, the rich power brokers will be back in business expanding government, immigration, the debt, taxes, & intrusion into private citizens lives, as they have for decades.