Here’s why the Democrats kept the Senate tonight:

And to ensure that the Senate could protect the people against themselves, the Framers armored the Senate against the people. …

And around the Senate as a whole there would be an additional, even stronger, layer of armor. Elections for senators would be held every two years, but only for a third of the senators. The other two-thirds would not be required to submit their record to the voters (or, to be more accurate, to their legislatures) at that time. This last piece of armor made the Senate a “stable institution” indeed. As a chronicler of the Senate was to write almost two centuries after its creation: “It was so arranged that while the House of Representatives would be subject to total overturn every two years, and the Presidency every four, the Senate, as a Senate, could never be repudiated. It was fixed, through the staggered-term principle, so that only a third of the total membership would be up for re-election every two years. It is therefore literally not possible for the voters ever to get at anything approaching a majority of the members of the Institution at any one time.”

— Robert Caro, Master of the Senate

If all 100 senators were up for election every two years, the Republicans would have romped tonight. But two-thirds of the Senate is immune to public repudiation in any particular election.

By the way, the link above takes you to the entire first chapter of Caro’s masterpiece. Essential reading if you want to understand the U.S. Senate.