Even though the Congress and President will be Republican, there is no guarantee they will always agree on policy. And that's not to mention the Democrats who will surely consolidate to push back on certain Trump policies.
2018 is going to be an absolute bloodbath for the Democrats, who never do well in mid-terms to begin with anyway. By that point, it's going to be closer to the UK Parliament after the next election - token opposition by Labour (Democrats in the US). And 2020 looks awful for the Democrats too - if Hillary is the best "standard" candidate, how is an inferior one going to win? Kanye West is probably the best choice for 2020 if Trump is anything to go by.
Even if they don't agree, look at how the Tea Party Caucus made life difficult for moderate Republicans and Paul Ryan - they will be emboldened by this. Republicans will largely fall-in line, offering maybe minor amendments or demands at most. Some, like Rand Paul, may stick to their principles, but quite frankly there are not many of those in the GOP. Look at what happened to John McCain!