If you’ve ever wondered how the U.S. Senate got to be the way it is today, this animated map might just answer that. It shows who held Senate seats from the early days of the republic right up through 2018. And it’s more than just a list of names—there’s a clear rhythm of change here, from early party dominance to the divided Senate we’re used to seeing now.
Let’s start with the basics. The Senate is one of the two chambers of Congress, and it’s been around since the first U.S. Congress met in 1789. Each state gets two senators, no matter how big or small it is. That part’s stayed the same since the beginning. What’s changed is how those senators are chosen and which political parties they belong to.
In the early years, senators weren’t elected by the public—they were picked by state legislatures. That changed with the 17th Amendment in 1913, which gave citizens the power to vote for senators directly. Since then, elections have been held on a rotating schedule: one-third of Senate seats are up for grabs every two years. That’s why you often hear people say the Senate is divided into three “classes.”
The animated map below, created by Reddit user u/SlimmPickens, does a great job of laying all of this out visually.
You can watch each state split in half—literally—because the map shows both Senate seats per state side-by-side. And every time one of those seats changes hands, the color changes to reflect the new party. It’s clean, clear, and fascinating to watch the ebb and flow of power over time.
What makes the video even more useful is that it doesn’t just show party control. It also highlights long-term vacancies—like those caused by civil war or prolonged disputes. Most short absences aren’t marked, but when a seat went unfilled for nearly a year or more, it’s shown in gray.
You’ll also notice that some familiar political roles don’t appear right away. For example, the Senate Majority Leader—the person who sets the legislative agenda for the chamber—doesn’t show up until the 1920s. That’s not an oversight; the title didn’t even exist before that. It only became an official leadership position during the early 20th century, when the structure of party politics in the Senate began to resemble what we see today.
The data behind the map comes from official Senate records, so it’s historically accurate. It covers all Senate elections and appointments from 1789 through 2018, pulling from publicly available historical data.
So what do you actually see when you watch it?
In the early years, you’ll spot the dominance of the Democratic-Republican Party (yes, that was a real party). Later, as the two-party system we know today began to take shape, you can see the rise of the Republican Party in the mid-1800s, especially after the Civil War. In the 20th century, party control bounced back and forth more frequently, with longer stretches of divided government and narrower margins.
New states entering the Union are also easy to spot—they pop into the map with two new seats, quickly filled by senators from whatever parties were popular in that state at the time. You can track how the balance of power gradually shifts as the country expands westward.
