The next US president's ability to govern and enact his or her policy agenda will depend in large part on which party wins control of both houses of Congress.

If a president's party controls both the House of Representatives and Senate, it is far easier to get legislation passed.

Here are some key races to watch that could tip the scales for Kamala Harris's Democrats or Donald Trump's Republicans: 

SENATE

Democrats currently have the slimmest of majorities in the 100-seat US Senate, with 47 senators and four independents who generally vote with them making up a 51-49 advantage. On Tuesday, 34 seats are up for grabs. 

The Republican Party is basically assured of flipping at least one seat -- that of West Virginia's Joe Manchin, a Democrat-turned-independent who is not running for reelection. Popular Republican Governor Jim Justice is expected to replace him.

Any other flip into the Republicans' red column would give them control of the chamber.

Montana

In Montana, a state that voted for Trump in 2016 and 2020, Democratic incumbent Jon Tester -- the only senator who is also a working farmer -- is in a complicated battle to keep his seat.

He is facing Republican Tim Sheehy, a retired Navy SEAL turned aerospace CEO who is currently ahead in opinion polls. Sheehy has faced accusations that he lied about being wounded in combat.

Ohio

Democratic incumbent Sherrod Brown, 71, has been in office as a senator from Ohio since 2007. He is hoping for another six-year team, but this state also voted for Trump in the last two presidential elections.

Brown is facing off against Republican Bernie Moreno, a 57-year-old onetime car dealer who was born in Colombia. The race is considered a toss-up. 

Brown has campaigned hard on abortion rights, after voters in 2022 cast ballots to ensure access to the procedure in the generally conservative state.

Pennsylvania

Bob Casey Jr, the Democratic incumbent, is running for reelection in what is considered the prize swing state in this year's election: Pennsylvania. 

Most polls show Casey with a slight lead over Republican businessman David McCormick, who has spoken at several Trump rallies, including the Butler event where a shooter tried to assassinate the former president in July. 

Casey's seat is still considered in play.

Michigan

Democratic Senator Debbie Stabenow is retiring, meaning this is an open seat.

Democrats are looking to Congresswoman Elissa Slotkin, a 48-year-old former CIA analyst, to keep the seat in the party's control. She takes on former congressman Mike Rogers, a onetime FBI agent who chaired the House intelligence committee.

In the latest polls, Slotkin is ahead but her lead is within the margin of error.

Wisconsin

In this key swing state, Democrat Tammy Baldwin, 62, is fighting for a third term in the Senate. In 2013, she became the first openly lesbian senator in history.

Baldwin has a slight lead over Republican rival Eric Hovde in most polls, according to Real Clear Polling, but that lead is largely within the margin of error.

Texas and Nebraska

While most toss-up Senate seats are currently in Democratic hands, Republicans are defending a few hard-fought seats as well. 

In Texas, veteran lawmaker and party heavyweight Ted Cruz is battling Democrat Colin Allred, a former NFL player. A Cruz loss would be devastating for the Republicans. 

In Nebraska, independent Dan Osborn is looking for an upset win against Republican incumbent Deb Fischer, who has served two terms in the Senate.

HOUSE OF REPRESENTATIVES

The House of Representatives is currently controlled by the Republicans. All 435 seats are up for grabs, as House members serve two-year terms. The magic number for control is 218 seats.

For Democrats to reach that bar, they must do well in New York and California.

In the Empire State, a mini red wave swept through in the 2022 midterms, with Democrats losing five seats they will hope to regain this time around, but the races are tight.