Could a tax on the rich be the political kryptonite that weakens Charlie Baker’s seemingly invincible status as one of the most popular governors in the country? Some Democrats seem to think so. Lawmakers on Wednesday voted to place a constitutional amendment calling for the so-called millionaire’s tax on the 2018 ballot, and Democrats immediately pounced on the issue as a political weapon that could be wielded against Baker and other Republicans running for state office next year.