Millions of Americans are heading to the polls as voting gets underway in the US midterm elections, with the results set to determine which party will control Congress for the final two years of Donald Trump's presidency.

The elections will decide the balance of power in both chambers of Congress: the House of Representatives and the Senate. Here is a basic guide to what is at stake in this year's midterm elections.

Trump's Republican Party currently holds control of both chambers, but Democrats are hoping to secure enough seats to take back a majority in the House, the Senate or both.

A Democratic victory in either chamber could significantly affect the president's ability to advance his legislative agenda. Gaining control of Congress would give Democrats greater power to challenge Trump's policies, block proposed legislation and scrutinise the actions of his administration.

Democrats could also use their congressional majority to launch investigations into the president and members of his administration. An attempt to impeach Trump could also become a possibility if Democrats gain control, according to BBC North America correspondent Anthony Zurcher.

Polling stations continue to open across the United States as voters cast their ballots. The latest states to begin voting include Idaho, California, Oregon and Nevada.

The first election results are expected to emerge later this evening in the United States, with the outcome likely to shape the political direction of the country and determine the balance of power in Washington for the remainder of Trump's presidential term.