How to know a red or blue wave when you see one
The 2026 midterm elections are around the corner. How will it compare to recent midterms?
Come midterm election season, media outlets and political pundits sometimes invoke a coming partisan "wave" — the idea that the conditions of Congressional races might have one party poised to gain serious ground on the other. With a week until the 2026 midterm elections, Republicans outnumber Democrats in the Senate 53–45 and in the House 218–214.
But what does a red or blue wave look like, and how can you spot one if it happens during the 2026 midterm elections next week?
There's no real quantifiable definition of a political wave, but past election data can help us understand the context.
Here's a look at the partisan composition of both chambers of Congress immediately after each election since the modern party system started (beginning with the 1856 election season), with the House in the darker shades and the Senate in the lighter shades. From this perspective, a blue wave would shift the bar to the left and a red wave to the right. It's worth noting that this doesn't include independent or third-party members or reflect shifts in the partisan composition between elections.
How balanced is Congress?
Partisan composition of both chambers of Congress after each election, 1856–2024
When were the biggest red and blue waves?
To examine the biggest waves, let's look at the shifts from one election year to the next. In the last 50 years, the largest wave came in the 2010 midterms, the first midterms of the Obama presidency, when Republicans gained 64 House seats and six Senate seats from the previous election. The largest blue wave in that time came in 2018, the first midterms under a Trump administration, when Democrats won 41 additional House seats despite ceding one in the Senate.
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Of course, a Republican win is usually a Democratic loss (and vice versa), so the size of the wave may be better expressed as a total swing in seats — otherwise expressed as a change in the margin between the majority and minority. In 2010, the red wave brought the House from a 79-seat Democratic majority to a 49-seat Republican majority, or a net change of 128 seats in the margin between the parties. It shifted the Senate 12 seats, from a 16-seat Democratic control to a four-seat Democratic majority.
The largest wave in the last 50 years was a red wave in the 2010 midterms.
Change in partisan seat margin of both chambers of Congress after each election, 1856–2024
Which elections have flipped the House and Senate?
The House of Representatives most often flips during midterms. Of the 19 majority shifts in the House since the modern party system started, 15 came during midterm years, including four of the last five midterms. The last time the House flipped during a presidential election year was in 1952.
Some chamber-flipping has come during historic partisan waves — such as in 2010 and 2018. Sometimes, though, it doesn't take a historical wave to flip control of a congressional chamber. In the 2022 midterms, Republicans collected a relatively modest 10 House seats and Democrats lost nine, but it was enough to turn the House red.
The Senate has had narrower margins in the 21st century, with the vice presidency deciding party majority on certain occasions (since the vice president is the deciding vote in a split House) and independents caucusing with one party or another deciding it at other times. New presidential administrations taking office and even special elections have flipped the chamber.
The House has flipped 19 times since the start of the modern two-party system.
Change in partisan seat margin of the House of Representatives in years that flipped majority control, 1856–2024
Keep reading about red and blue politics, swing states, and more, and get the data delivered with our weekly newsletter.
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- What are the current swing states, and how have they changed over time? - Five states were won by less than 3 percentage points in the 2024 presidential election.
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- Reapportionment and redistricting after the 2020 census: Explained - Texas gained two congressional seats while seven states lost a seat following the 2020 census. In all, seven seats shifted between 13 states.
