As Democrats strategize on how to stop conservative legislation from making its way to Donald Trump’s desk in the White House, Republicans have a little-known rule in their toolbox that would allow them to pass legislation, including a repeal of Obamacare, with a simple majority. Democrats were able to keep 48 seats in the Senate after Tuesday’s election, giving the party the power to filibuster legislation and effectively prevent conservative policies from being enacted. But Republicans can turn to a seldom-used Senate rule that would allow them to pass legislation by a simple majority vote—legislation that has a greater chance...