Will the Republicans be able to take control of the Senate after the November election?
Some form of this question is asked every two years when one-third of the Senate membership comes up for reelection (it also asked of the House of Representatives).
Despite the hopes of parties and politicians, even with the best polling, no one can predict with absolute certainty the outcome of any election. But data can show us what has happened. Below is a graph illustrating the partisan control of the Senate on the first day of each Congress since the first one, according to data from the Senate website.
The change to direct election of senators in 1913, first reflected in the 1914 election, doesn't appear to have had much effect, at least in terms of parties other than the Democrats or Republicans attaining control of the chamber. The two main parties were already in control of the state legislatures. Also, the first Republican Party below is not the current, modern Republican Party.
The vertical axis represents the percentage control of the Senate that each party enjoyed immediately following each election; it doesn't take into account Vermont Sen. James Jeffords's switch from the GOP to being an independent in 2001, for example, five months after the start of the 107th Congress. The horizontal axis is the number of each Congress. For details, hover the cursor over each line; to highlight a party, click on that party in the legend. Party designations are those of the Senate website.
Are the first several Congresses hard to read? Parties ranging from the Anti-Jacksons to the Nullifiers came into existence before largely disappearing as ideological positions shifted, or larger parties adopted the smaller parties' positions. Below is a close-up of those first Congresses.