This map shows where the Senate and House majorities could be won and lost

A string of races where polls close at 8 p.m. ET on Tuesday will likely play a massive role in the fight for control of the Senate and House.

Follow the most up to date results for key races here.

Results in key races will start coming in after 6 p.m. ET, and Democrats' fortunes in early reporting states will give some clues into how successful they can be in red territory in this year's midterm elections. The results that will come in after 8 p.m., though, will make or break Democrats' chances of taking control of Congress.

The contests will determine whether one of the most vulnerable Senate Democrats can win re-election — or if the party can pull upsets in two of the only states where it has a chance to win a GOP-held seat. Meanwhile, Democrats could all but wrap up a House majority — or come closer to failure in reaching their goal of flipping a net 23 GOP-held seats — in races across Pennsylvania, New Jersey and several Midwestern states.

Here are the key races to watch at 8 p.m. ET:

Senate

Scott Olson | Getty Images

Senator Claire McCaskill (D-MO) is greeted by people waitng to vote as she leaves the polling station after voting on November 6, 2018 in Kirkwood, Missouri. 

Polls will close at 8 p.m. ET for several of the most competitive Senate races, and results will start coming in shortly after. If these contests break heavily for one party or the other, so would that party's chances of controlling the chamber.

• Missouri: Democratic Sen. Claire McCaskill aims to win re-election against Republican Missouri Attorney General Josh Hawley in a state that President Donald Trump won by about 20 percentage points. A Hawley victory would cut into Democrats' already slim hopes of winning the Senate. The race was too close to call as of 8 p.m. ET, according to NBC News.
• Tennessee: Republicans aim to defend this red state after Republican Sen. Bob Corker's retirement. Democratic former Gov. Phil Bredesen's centrist campaign has made the race closer than expected, but pro-Trump GOP Rep. Marsha Blackburn was the favorite entering Election Day. A Bredesen upset could put a majority in play for Democrats. The contest was too early to call as of 8 p.m. ET, with Blackburn leading, according to NBC News.
• Texas: Along with Tennessee, Texas offers Democrats a chance to steal a seat in red territory. Democratic Rep. Beto O'Rourke has leveraged liberal energy and record-breaking fundraising to put Republican Sen. Ted Cruz on his heels. Still, the incumbent was the favorite heading into Tuesday's election.
• Florida: Polls in most of Florida will close at 7 p.m. ET, giving an early picture of who will win the high-profile contest between Democratic Sen. Bill Nelson and Republican Gov. Rick Scott. When the remainder of polls close at 8 p.m. ET in the red Panhandle, it will give a more complete picture of who could win the most expensive Senate race of the 2018 cycle. The race was too close to call as of 8 p.m. ET, according to NBC News.

• New Jersey: Polling suggests Democratic Sen. Bob Menendez has recently pulled away from Republican challenger Bob Hugin, but the scandal-wracked incumbent is still on upset alert. A Democratic loss in New Jersey would virtually end the party's hopes for Senate control, regardless of whether McCaskill wins.
• Mississippi: Democratic former Agriculture Secretary Mike Espy and Republican state Sen. Chris McDaniel aim to unseat GOP Sen. Cindy Hyde-Smith, who was appointed to replace the retired Republican Sen. Thad Cochran. If no candidate wins 50 percent of the vote, it will go to a runoff later this month. Polling suggests Hyde-Smith and Espy will make a runoff, with the incumbent a heavy favorite to keep her seat. The contest was too early to call as of 8 p.m. ET, according to NBC News.

Earlier polls that close before 8 p.m. will also help determine the outcome. Democratic Sens. Joe Donnelly and Joe Manchin are defending their seats in red states Indiana and West Virginia, respectively. The Indiana and West Virginia races were too close and too early to call, respectively, according to NBC News.

While polls in several key Senate races — for the GOP-held seats in Arizona and Nevada and the Democratic-held seats in North Dakota and Montana — will not close until later, the results after 8 p.m. ET could all but decide Senate control.

House

Gene J. Puskar | AP

Rep. Conor Lamb, D-Pa, who is running against Rep Keith Rothfus, R-Pa in Pennsylvania's 17th Congressional District, exits his polling place after voting Tuesday, Nov. 6, 2018 in Mt. Lebanon, Pa. 

In addition, polls close in several of the most important House battleground states at 8 p.m. ET, after which results will start trickling in. These contests will likely determine whether a Democratic House majority can be called early — or if the GOP has a chance to defend its control of the chamber on the West Coast later in the night.

Democrats need to gain a net 23 Republican-held House districts to win a majority and could have already picked off some districts in Florida, Virginia, Kentucky or Ohio earlier in the night.

• Pennsylvania: Redistricting has made swing state Pennsylvania one of the most fascinating and competitive states in this year's midterms. The nonpartisan Cook Political Report considers eight Keystone State House seats either likely to flip or highly competitive. Republicans hold seven of those districts, while Democrats control one.
• New Jersey: Democrats also have an opportunity to pick up several GOP-held seats in New Jersey. Cook lists four New Jersey districts controlled by Republicans as either toss-ups or likely to flip.

• Texas: Four seats in Texas, all controlled by the GOP, are considered competitive on Tuesday night.
• The Midwest: Various Midwestern states could lead the House battle to swing one way or another. The competitive GOP-held districts include four in Illinois, three in Michigan, two in Kansas and one in Missouri.