Why is Mitt Romney coming after Trump?

Who does Mitt Romney support, and why is he coming after Trump? Does he have an altruist motive that he really fears Trump will do real damage to the country? Is someone paying him off? What exactly is it? I didn’t see or hear all that Romney has said, I’ve only heard on the news he is challenging Trump about his taxes, and I guess trying to turn the wave away from carrying Trump to the finish line.