There are two elements to votes on during today's midterm elections -- the election of members of the House of Representatives and the election of members of the Senate.
These two branches control the Congress. The Republicans and Democrats are vying for a majority of both the House and Senate.
What happens if the Republicans win the House?
The House of Representatives is made up of 435 members, and all are up for election in today’s vote.
To achieve a majority either party would 218 or more seats.
Should the GOP win control of the House, passing legislation would remain in a comfortable position for President Donald Trump.
If the Dems achieve a majority, Trump would find his power lessened as any new bills or amendments to current bills would first have to be approved by the House.
What happens if the Republicans win the Senate?
The Senate shares with the House of Representatives the responsibility for all lawmaking within the United States.
For an act of Congress to be valid, both houses must approve an identical document.
Senators have the power to decide on major presidential appointments, and also should an impeachment trial come to pass it occurs in the Senate.
As well as this, treaties with other nations must be agreed by two-thirds of the Senate.
There are 33 of the 100 seats up for election today, and two seats being contested in special elections.
Polling sites show it 80.5 percent likely that the Republicans will keep control of the Senate.
If the Republicans achieve a majority, they could still approve federal judges and the President’s Supreme Court nominees. However, if they want to get any big legislation approved the Senate and the Democrats have the House majority, they will have have to negotiate with Democrats in the House of Representatives.