The water had been rising from the moment Democrats recaptured the House of Representatives. In September, the impeachment dam finally burst. Now there is no telling who will drown in the coming flood or even which political party will find itself most squarely in its path. President Trump launched a full-court press against former Vice President Joe Biden, a leading Democratic presidential candidate. Trump has clearly calculated that exposing the Biden family's dealings in Ukraine will either weaken the veteran Democrat as a general election opponent or sink him altogether, paving the way for a more liberal challenger. Meanwhile, Democrats have decided impeachment is a risk worth taking. Both parties are gambling, pushing all their chips to the middle of the table. House Speaker Nancy Pelosi declared herself “heartbroken” over her decision to launch an impeachment inquiry. “This is a very sad time for our country,” the California Democrat said. “There is no joy in this. We must be somber.” Among the activists upon which her young Democratic majority depends, however, Michigan Rep. Rashida Tlaib expresses the prevailing sentiment: “Impeach the motherf---er!” Unlike the eager Democratic freshmen who see themselves as analogous to the 1974 “Watergate babies,” Pelosi was genuinely reluctant to go down this path. Her frame of reference is not the downfall of Richard Nixon, but rather the failed attempt to remove Bill Clinton 20 years ago. Republicans held the House and could...