By John Whitesides MILWAUKEE (Reuters) - Democratic presidential candidate Hillary Clinton accused rival Bernie Sanders in a debate on Thursday of misleading Americans about the costs and viability of his healthcare plan, saying he was making promises "that cannot be kept." In their sixth presidential debate, Clinton said Sanders' proposal for a single-payer, Medicare-for-all healthcare plan would mean dismantling Obamacare and triggering another intense political struggle. "You need to level with people about what they will have at the end of the process you are proposing," Clinton said. "That's a promise that cannot be kept." Sanders said he would not dismantle the healthcare plan known as Obamacare and was simply moving to provide what most industrialized countries have - healthcare coverage for all.