Jeff Greenfield explains at Politico why Democrats hoping for a successful 2018 midterm election should take a closer look at history.

There are two assertions about midterm elections that are endlessly recited with firm confidence. First, the president’s party almost always loses seats in Congress in midterm elections, and second, the fate of the president’s party is directly tied to his or her approval ratings. Yet there are rarely mentioned, but significant, caveats to these putative iron laws of politics.

The first is that aggregate numbers can be misleading. If you do the math, you find that in the last 21 midterms, the party holding the White House lost an average of 31 seats. But this is like saying the average wealth of Jeff Bezos and nine insolvent people is $10 billion. The “average midterm loss” conflates massive defeats with insignificant changes in the congressional landscape that left the president little or none the worse for wear—and in a few cases even improved his lot. And second, it obscures the fact that there are very different reasons for some of the biggest defeats suffered by a president’s party. When the president gets a “shellacking,” as President Barack Obama memorably put it after Democrats’ 2010 midterm wipeout, it’s not always obvious his unpopularity is the sole or dominant reason. …

… Look back further, though, and you find plenty of examples in which presidents endured only a political flesh wound, if that. In 1962, the Democrats lost only four House seats and picked up two Senate seats. In 1970, the Republicans lost only 12 House seats and gained two Senate seats. In 1990, George H.W. Bush saw his party lose only eight House seats and one Senate seat. And under Bill Clinton in 1998 and George W. Bush in 2002, the party in power gained seats in the House.