For this year’s midterm elections, Democrats have history on their side: The sitting president’s party has gained seats in both houses in just two midterms in the last 150 years.

Recent midterm waves -- where the opposing party picked up significant numbers of House seats -- tended to share five common themes: A low presidential approval rating; intensity in the opposition party; success in special elections; a lot of retirements in the majority party; and a weak economy.

Democrats are certainly expected to pick up some seats in 2018. But Republicans have a substantial majority in the House and a strong economy.

So which party will prevail?

In the video above, we explore conditions that are ripe for a “blue wave” that could enable Democrats to take back the House and Senate, or at least wash away some of the Republican power in Washington.

Write to Aaron Zitner at Aaron.Zitner@dowjones.com