AUGUSTA, Maine — Gov. Paul LePage has signed onto a Texas-led lawsuit to overturn the Affordable Care Act, saying it’s unconstitutional now that Congress has moved to repeal the key tax at the heart of the health care law enshrined by former President Barack Obama. The Republican governor and 19 Republican-led states filed the lawsuit on Monday in a Texas federal court against the administration of President Donald Trump — an Obamacare opponent — alleging that the 2010 law now “forces an unconstitutional and irrational regime” onto states. It revolves around the so-called “individual mandate,” a key provision in the Affordable Care Act making Americans have health insurance or face a penalty.