With key Obamacare tax credits set to expire within weeks, Democrats have unified behind a simple message: extend the subsidies and keep health insurance premiums from spiking for more than 20 million Americans. Republicans, meanwhile, have engaged in a wide-ranging blame game while scrambling to coalesce behind an easily digestible plan to lower health care costs. That struggle comes to a head this week as House leaders move to put what they hope will be a consensus GOP plan up for a vote.