Democrats have swept key US midterm races, flipping long-held Republican seats and securing a majority in Congress as voters rebuked Donald Trump's policies at the ballot box.

In an unexpected performance, Democrats defeated incumbent Republicans candidates for historically conservative Senate seats in Iowa, Kansas, North Carolina, South Carolina and Texas.

Polls indicated Democrats were leading in competitive races but failed to fully predict the extent of the party's massive wins. Voters have expressed disapproval of Trump's performance as president, especially related to the cost of living.

Despite telling voters to act as though he was still on the ballot, Trump has dismissed concerns the results are a response to his presidency, saying he would have "won in a landslide" had he been on the ballot.

Here are the key takeaways from the results: