On Dec. 24, 2009, the Democratic-controlled Senate passed President Obama’s healthcare law with a filibuster-proof 60-vote majority, triggering a massive backlash that propelled Republicans to control of the House the following year. On the Senate side, going into Tuesday’s elections, 24 senators who voted for Obamacare were already out or not going be part of the new Senate being sworn in on January. To be sure, it isn’t fair to attribute all of the turnover in the chamber to Obamacare. Many senators voted for Obamacare and lost re-election battles in which they were hit hard for their support for the...