I think he would be more effective staying a Republican. It won't be Trump's party forever and the GOP will be looking to distance themselves from 45. Not that I am that concerned about the Republican party, though ;)
the Hill: Rep. Justin Amash (R-Mich.) on Thursday announced he's leaving the Republican Party and becoming an independent, months after igniting a political firestorm when he became the first sitting GOP congressman to call for an impeachment inquiry against President Trump.
"Today, I am declaring my independence and leaving the Republican Party," Amash wrote in a Fourth of July op-ed for the Washington Post. "No matter your circumstance, I’m asking you to join me in rejecting the partisan loyalties and rhetoric that divide and dehumanize us. I’m asking you to believe that we can do better than this two-party system — and to work toward it. If we continue to take America for granted, we will lose it."