There are 35 U.S. Senate elections today. After tonight, either Republicans or Democrats could end up with a majority. Currently, Republicans hold 51 seats and Democrats hold 49 seats (technically, one is an incumbent).

As results come in after polls close tonight, there are plenty of ways to keep score in the race for Senate control. Here's the simplest way to look at it:

Republicans have 46 seats locked up. There are 16 potentially competitive Senate races. Republicans need to win 4 of those 16 in order to keep control of the Senate. (In a 50-50 tie, the vice president can break the tie).

Some more detail:

About two-thirds of the Senate (65 seats) is not up for re-election this year. Forty-two of those 65 are Republicans. So before a ballot is cast, Republicans have 42 seats. Not bad.

Four of the 9 GOP seats up this year are safe: Sens. Roger Wicker of Mississippi, Deb Fischer of Nebraska, and John Barrasso of Wyoming, plus Mitt Romney running to replace Orrin Hatch in Utah. That puts the GOP at 46 safe seats, four away from a functional majority.

At the bottom are the 16 races that are potentially competitive, by closing time, Eastern Standard Time. In states with multiple time zones and multiple closing times, I've listed the latest closing time.

If the GOP wins just four of these races, whether currently held by a Republican or not, they have 50 senators.

First, here's the map, with the 16 competitive races in olive green:



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7:00 p.m. EST

Indiana: Sen. Joe Donnelly is arguably the most vulnerable Democrat, facing Republican Mike Braun.



7:30 p.m. EST

Ohio: Democratic Sen. Sherrod Brown is a strong favorite for re-election, but he is not totally safe.

West Virginia: Democratic Sen. Joe Manchin is vulnerable, but favored, over Republican Patrick Morrisey.



8:00 p.m. EST

New Jersey: Democratic Sen. Bob Menendez has faced federal corruption charges and could lose.



9:00 p.m. EST

Arizona: Open seat being vacated by Republican Sen. Jeff Flake. It's a top Dem pickup opportunity.

Florida: Democratic Sen. Bill Nelson looked safe until recently. Republican Florida Gov. Rick Scott could knock him off.

Michigan: Democratic Sen. Debbie Stabenow is favored, but vulnerable.

Minnesota: Appointive Democratic Sen. Tina Smith could lose.

Mississippi: Mississippi has two Senate elections today. One is noncompetitive (Sen. Roger Wicker will win easily). Republican Sen. Cindy Hyde-Smith, appointed to fill out the term of Sen. Thad Cochran, is running in a special election. There are four candidates, Hyde-Smith would need a majority of the vote in order to avoid a Nov. 27 runoff.

Missouri: Sen. Claire McCaskill is a fighter, but her state is trending away from Democrats. She is one of the most vulnerable senators for sure this year.

Tennessee: This is an open seat where Republican Marsha Blackburn is a slight favorite over Democrat Phil Bredesen.

Texas: Republican Sen. Ted Cruz is fighting off a very well funded challenge by media darling Democratic Rep. Beto O'Rourke.

Wisconsin: Democratic Sen. Tammy Baldwin is favored over Republican challenger Leah Vukmir.



10:00 p.m. EST

Montana: Democratic Sen. Jon Tester is definitely vulnerable.

Nevada: Republican Sen. Dean Heller is in a very tough re-election battle.

North Dakota: Democratic Sen. Heidi Heitkamp is well-funded but vulnerable.

[Also read: The 10 key races to watch: How to know if you're witnessing a blue wave]