President Trump ended 2019 with a strong economy, a string of domestic policy achievements—and the notoriety of becoming the third president to be impeached by the House.

The mix of political highs and lows that has characterized Mr. Trump’s presidency was especially pronounced in his third year. He welcomed a special counsel report that found insufficient evidence the Trump campaign conspired with Russia to interfere in the 2016 election, though it drew no conclusion about whether the president obstructed justice. But his...