It was summer, and a major U.S. political party had just chosen an inexperienced, unqualified, loutish, wealthy outsider with ambiguous party loyalties to be its presidential nominee. Some party luminaries thought he would help them win the general election. But many of the faithful were furious and mystified: How could their party compromise its ideals to such a degree?

Sound like 2016? This happened a century and a half ago.

Many have called Donald Trump’s unexpected takeover of a major political party unprecedented; but it’s not. A similar scenario unfolded in 1848, when General Zachary Taylor, a roughhewn career soldier who had never even voted in a presidential election, conquered the Whig Party.

A look back at what happened that year is eye-opening—and offers warnings for those on both sides of the aisle. Democrats quick to dismiss Trump should beware: Taylor parlayed his outsider appeal to defeat Lewis Cass, an experienced former Cabinet secretary and senator. But Republicans should beware, too: Taylor is often ranked as one of the worst presidents in U.S. history—and, more seriously, the Whig Party never recovered from his victory. In fact, just a few years after Taylor was elected under the Whig banner, the party dissolved—undermined by the divisions that caused Taylor’s nomination in the first place, and also by the loss of faith that followed it.

Born in 1784 into a prominent Southern slaveholding family, Taylor was commissioned as an army officer at age 23. He first distinguished himself as a captain in the War of 1812 and gained even greater fame in the Second Seminole War, for which he earned the nickname “Old Rough and Ready” by bravely crossing a treacherous swamp with his men during the Battle of Okeechobee. The moniker suited this stocky, stern, undisciplined slob, who shared his men’s battlefield hardships and rarely dressed in military finery. With his signature straw hat, “he looks more like an old farmer going to market with eggs to sell,” one officer muttered. ...