DONALD Trump has described the midterm election result as a “tremendous success,” despite his Republican party losing control over the House of Representatives.

“Tremendous success tonight. Thank you to all!” the US President said on Twitter, his only remarks after projections came in.

The House of Representatives will flip to the control of the Democrats while Trump’s Republicans have retained control over the Senate.

The projections are largely in line with polling ahead of election day on Tuesday, which predicted a split Congress, and would likely be a check on the president's agenda.

White House spokeswoman Sarah Huckabee Sanders earlier minimised likely Democratic gains.

“Maybe you get a ripple, but I certainly don't think that there's a blue wave,” she told reporters, pointing to several early Republican wins.

Should Republicans lose the House, Sanders said the president's agenda was not going to change.

As for Republicans retaining control of the Senate, she called it “a huge moment and victory for the president.”

Despite Trump's deeply polarising effect on voters, the GOP harnessed his rock-solid popularity with hard-right voters in rural, deep-red states.

They held onto seats in the South, Midwest and West and ensured at least a 50-50 Senate - enough to give Republicans control because Vice President Mike Pence as president of the Senate would tip the balance to the GOP.

Republicans paved their path to victory by defeating Democrats Joe Donnelly of Indiana and Heidi Heitkamp of North Dakota. They also held on to competitive seats in Texas, where Sen. Ted Cruz fended off Republican Beto O'Rourke, the well-financed liberal darling, and in Tennessee, where Republican Marsha Blackburn prevailed.

The White House has been stressing the historical headwinds it faced: In the last three decades, 2002 was the only midterm election when the party in office gained Senate seats. And only twice in the past eight decades has the president's party picked up House seats in the midterms.

Trump spent election night watching returns with family and friends at the White House, his shadow looming large over the results.