Whether you want to call it the "Kavanaugh effect" or just chalk it up to Republican voters returning home as the election approaches, it's become clear that Democrats' chances of taking over the U.S. Senate have gotten slimmer.

As of Wednesday afternoon, elections forecaster Nate Silver's site FiveThirtyEight gives Republicans a better chance of reaching 54 seats or more than he gives to Democrats for taking over the majority.

Specifically, as of this writing, the site's "classic model" gives Republicans a 20.3 percent chance of reaching at least 54 seats, which compares with an 18.4 percent chance of Democrats taking over control of the Senate.

Republicans entered this year with an inherent advantage. Because Vice President Mike Pence can be a tiebreaking vote, Democrats need to win 51 seats to take over the chamber while Republicans need to hold just 50 seats to maintain control. Furthermore, Democrats are defending more seats, and the Senate map is heavily tilted toward red states.

Though Republicans were always favorites to keep the Senate, their odds have improved in recent weeks, with three states in particular giving them a boost. Republicans are now considered "likely" to keep their seats in Texas and Tennessee, and North Dakota seems ready to flip into the Republican column. Barring any other major upsets, victories in those three races would be enough for Republicans to keep the Senate — hence their 81.6 percent chances overall.

To get to 54, the most likely scenario would be that Republicans win the toss-up states of Nevada and Missouri, and then surge to victory in Arizona and Florida (two races that are currently tilting Democrat, but well within range of Republican victory). Beyond that, they'd have to start flipping some seats that are currently considered "likely" to remain Democratic, such as Montana and West Virginia.

The Senate forecast is the mirror image of the House forecast, where Republicans are defending seats in dozens of districts that Hillary Clinton won in 2016 or where President Trump is unpopular. The current forecast gives Democrats a 83.8 percent chance of flipping control of the House, compared to the 16.2 percent chance that Republicans keep it.

These are only probabilities, however, and one in five events happen frequently enough that it's far from a sure thing.