"This is as important to us as a presidential campaign," said Mary Kay Henry, president of Service Employees International Union, whose 2 million members played a central role in helping pass Obamacare and are expected to be critical in defending it.
The order is daunting.
Republicans, who will control both the White House and Congress for the first time in more than decade, credit their victories in part to a relentless campaign against Obamacare.
And with some Americans struggling with large insurance premiums, GOP lawmakers have had no trouble finding horror stories to bolster their repeal effort.
"Obamacare is ripping apart at the seams, and things are only getting worse," Rep. Diane Black, R-Tenn., said in the party's weekly radio address. The Tennessee insurance market has experienced some of the worst turmoil in the country over the last year.
At the same time, Democrats have taken a hands-off approach to the law in recent years, wary of being linked to its struggles.
But as this new chapter in the health care debate begins, they have some key advantages.
"Democrats who were always a little squirrely on robustly defending the Affordable Care Act are on very firm ground in fighting repeal," said Sen. Christopher S. Murphy, D-Conn.
Though public opinion about Obamacare is still split, most provisions of the law are extremely popular, even with Republican voters. That may fuel a major backlash if the GOP moves to take them away.
Eight in 10 Americans in another poll say they like provisions in the law that eliminate out-of-pocket costs for many preventive services such as cancer screenings or provide federal aid to states so they can expand Medicaid coverage for poor patients.