Mitt Romney's campaign is aligning itself with President Barack Obama—and breaking from Republican leaders—by saying the government will be imposing a penalty, not a tax, on people who don't buy insurance as required by the new health-care law. The break from his Republican allies illustrates the difficulty the presumptive GOP presidential nominee faces in criticizing the president for a national health-care law that resembles the one Mr. Romney signed as Massachusetts governor. Both laws include a requirement that most individuals buy insurance coverage.