WASHINGTON— Former U.S. Rep. Justin Amash of west Michigan said Thursday he’s forming an exploratory committee as he mulls joining the Republican primary contest for the state’s open U.S. Senate seat. Amash, 43, of Cascade Township is a fierce critic of former President Donald Trump who left the GOP in 2019 and later became the first Libertarian to serve in Congress. He chose not to run for reelection in 2020 after five terms and mounted a brief exploratory bid for the presidential race. Amash said Thursday the Senate needs a “principled, consistent constitutional conservative” who will take o…