Less than a month from Election Day, both parties have been focused on registering as many voters as possible and competing to see which holds the advantage in turning out their base. The turnout battle is the most important fight in winning control of Congress.

Since the November 2016 presidential election, over 25 million people have registered to vote for the first time or have used a registration form to update their address, name or party affiliation.

From the general election in 2014 to the general election two years later, roughly 40 million new voters registered. Taking into account different midterm and presidential electorates, the number of new voters from 2016 to now is sizable, according to data provided by TargetSmart and independently analyzed by the NBC News Data Analytics Lab.

Including both those who registered for the first time and those who updated their registration form helps measure the number of people who have shown enthusiasm about the upcoming election.

Among the people who have registered to vote for the first time or who have updated their registration forms since the 2016 presidential elections, 40 percent are Democrats and 28 percent are Republicans. The remaining 32 percent are those who registered with a third party or are not registered with any party, as is common in states with open primary elections.

Party identification nationwide numbers are based on a combination of state-provided partisanship (when available) and TargetSmart's model of party affiliation.

While determining turnout prior to Election Day is not possible, it seems the Democrats hold a nationwide advantage in terms of new or updated voter registration from the November 2016 presidential election to now.

However, the number should not be misinterpreted — the percentage of registered Democrats and Republicans from 2016 to now is identical to registrations from November 2014 to November 2016.

Comparing registration activity from 2014 to 2016 with activity from 2016 to now helps put these numbers into context, but it should be noted that 2014 and 2016 include a complete picture of registration from those years, whereas 2018 only includes registration activity up until this point in time.

The NBC News Data Analytics Lab will focus on seven key Senate races — Arizona, Florida, Indiana, Montana, Nevada, Tennessee and Texas — and continue to track and analyze voter registration activity as well as early and absentee voting in those states in the coming weeks.

The seven Senate races will prove pivotal in determining which party controls the Senate, and neither party has enough momentum in registration activity in those states to say for sure which holds an advantage.

- In Montana, where Democratic Sen. Jon Tester is up for re-election in a state that President Donald Trump won by 21 points, Democrats have made up 42 percent of registration activity since November 2016 compared to 30 percent for Republicans. In comparison, from 2014 to 2016, Democrats only made up 22 percent of registration activity in Montana and Republicans made up 37 percent, indicating that Democrats have some momentum in Montana this year. Tester faces Republican Matt Rosendale next month.

-In Texas, where Republican Sen. Ted Cruz is in a tight race against Democratic Rep. Beto O'Rourke, Democrats have made up 41 percent of registration activity compared to 40 percent for Republicans. Those partisan numbers are a slight gain for the Democrats from 2014 to 2016, when they made up 38 percent of registration and Republicans made up 42 percent.

- In Nevada, where Republican Sen. Dean Heller is up for re-election in a close race against Democratic Rep. Jacky Rosen, Democrats have made up 34 percent of registration activity since November 2016 compared to 30 percent for Republicans. This is a small increase for Republicans and a small decrease for Democrats across the two election cycles — from 2014 to 2016, Democrats made up 37 percent of registration activity and Republicans made up 27 percent.

- In Florida, where Democratic Sen. Bill Nelson is running for re-election in a tight race against Republican Gov. Rick Scott, Democrats have made up 29 percent of registration activity compared to 28 percent for Republicans. Democratic registration is down slightly from 2014 to 2016, when they made up 32 percent of registration activity. Republicans were at the same 28 percent then.

- In Indiana, where Democratic Sen. Joe Donnelly is facing a re-election challenge from Republican businessman Mike Braun, a larger percentage of registration activity has come from Republicans (41 percent) than Democrats (33 percent). From 2014 to 2016, a larger percentage of registration activity came from Democrats (36 percent) than from Republicans (34 percent).

Republicans also have higher registration numbers than Democrats in two key states with opportunities for Democratic pickups — Arizona and Tennessee.

- In Arizona, where two members of the House, Democrat Kyrsten Sinema and Republican Martha McSally, are in a neck-and-neck contest to fill Republican Sen. Jeff Flake's seat, Republicans have made up 28 percent of registration activity from November 2016 to now, compared to 27 percent for Democrats. From 2014 to 2016, Democrats had a minor advantage — Republicans made up 26 percent of registration activity and Democrats made up 27 percent.

- In Tennessee, where former Democratic Gov. Phil Bredesen is in a dead heat with Republican Rep. Marsha Blackburn to fill Republican Sen. Bob Corker's seat, Republicans have made up 49 percent of registration activity compared to 27 percent for Democrats. From 2014 to 2016, Republicans also had a higher registration numbers. They made up 52 percent of registration and Democrats made up 29 percent.

Data analysis conducted by the NBC News Data Analytics Lab in conjunction with the Penn Program on Opinion Research and Election Studies (PORES). Voter file data, collected by TargetSmart, contains information on most voters’ turnout history and selected demographic information. Arizona data is current as of Sept. 11, Florida data is current as of Aug. 31, Indiana data is current as of Oct. 6, Montana data is current as of Sept. 18, Nevada data is current as of Sept. 24, Tennessee data is current as of Sept. 14 and Texas data is current as of Sept. 30.