The Affordable Care Act: The president can stop repeal of Obamacare, but a determined congressional majority can wreak havoc by using the initial budget process, known as reconciliation, which allows major changes to be made with only a majority Senate vote that isn’t subject to veto.

“The Republicans can use reconciliation to pass lots of policies — even repealing parts of Obamacare,” says Lanhee Chen, formerly a top policy adviser to Mitt Romney and now a research fellow at Stanford University’s Hoover Institution and a Bloomberg View columnist.

Republicans have struggled to come up with any palatable alternatives to the health-care law. That means major components such as the subsidies and tax credits, or the ban on discrimination against insuring those with pre-existing conditions, wouldn’t be changed. But deep cuts in funding for running the program and getting new enrollees would take a toll.

“There could be a big hit in day-to-day administration,” says Chris Jennings, a health-care expert who has worked with the Obama administration.