Senate Majority Leader Mitch McConnell, flanked by Sen. Cory Gardner (R-CO), Sen. John Barrasso (R-WY), and Sen. John Thune (R-SD), speaks to the media after the weekly policy luncheon on Capitol Hill May 2, 2017 in Washington, DC.

Senate Republican leaders are shifting gears in their health care crusade, dropping their existing bill and instead trying to force a vote on a bill from 2015 that would repeal large swathes of Obamacare without any plan to replace it.

But the path to voting on that bill is an obstacle course. It could fail at multiple points — on the initial procedural vote or on final passage. An unpredictable amendment process could send the bill in unexpected directions. Clean (partial) repeal sounds simple, but the Senate procedure makes it anything but.