Pot legalization in the U.S. seems both inevitable and impossible

Some top Democrats in the Senate, led by Majority Leader Chuck Schumer, unveiled a proposal earlier this month to end the federal prohibition of marijuana and expunge the records of those with nonviolent pot-related offenses. The bill faces steep odds of passing through a divided Congress, but the simple fact that some of the most prominent lawmakers in Washington have backed the plan shows how dramatically views on pot have changed in a short period of time.

Over the past two decades, marijuana legalization has transformed from a fringe issue to...