Democrats suffered a further double blow in the US elections as Republicans retained control in both houses of Congress.

Republicans held on to a slim majority in the Senate, in a stinging blow to Democrats, who had been nearly certain of retaking control.

They saw their hopes fizzle out as endangered Republican incumbents won in Missouri, Pennsylvania, North Carolina and even Democrat-friendly Wisconsin.

Read more: US Election - Donald Trump vows to be a 'president for all Americans' after shock victory

Meanwhile, Republicans retained their lock on the House of Representatives for two more years as candidates triumphed in a variety of districts Democrats had hoped to take in Florida, Virginia and elsewhere.

Republicans started the night with a 54-46 majority in the Senate and are on track to end up with at least 52 seats, presuming they win a December run-off in Louisiana, as expected.

Senate majority leader Mitch McConnell, of Kentucky, who will serve in that role next year, issued a statement congratulating Mr Trump.

"After eight years of the Obama administration, the American people have chosen a new direction for our nation. President-elect Trump has a significant opportunity to bring our nation together. It is my hope and intent that we succeed in the years ahead by working together with our colleagues across the aisle to strengthen our national and economic security."

Republicans celebrated their wins by already looking ahead to mid-terms in 2018, when Democrats could see their numbers reduced even further with a group of Senate Democrats on the ballot. The party faces being consigned to minority status on Capitol Hill for years.

Read more: US Election - Donald Trump vows to be a 'president for all Americans' after shock victory

As the night wore on, Democratic officials struggled to explain why their optimistic assessments of retaking Senate control were so mistaken. Some blamed unexpected turnout by certain segments of white voters, or FBI director James Comey's bombshell announcement that he was reviewing a new batch of emails connected with Hillary Clinton.

The Senate races were shadowed every step of the way by the polarising presidential race between Mrs Clinton and Mr Trump, and in the end, he was apparently not the drag on Republican candidates that was widely anticipated.

Even though the party's renewed control of the Senate will be narrow, the advantages of being in the majority are significant. The controlling party holds the committee chairmanships, sets the legislative agenda and runs investigations. First up is likely to be a nominee to fill a vacancy on the Supreme Court.

In the lower chamber of Congress, Mr Trump's divisive comments about women and Hispanics meant Democrats had envisioned big gains in suburban and ethnically diverse districts.

Instead Democrats made modest pick-ups as Republican contenders were buoyed by their presidential candidate's battle against Ms Clinton.

Expectations had been low that Democrats would win the 30 seats they needed to capture House control, but both sides had anticipated they would cut the historic Republican majority by perhaps a dozen seats. Republicans currently hold a 247-188 majority, including three vacant seats, the most the party has commanded since their 270 in 1931.

House speaker Paul Ryan, the Republican representative for Wisconsin who won a 10th term, called Mr Trump to congratulate him on his win, and pledged to work together after a fractious relationship before the vote.

"We are eager to work hand-in-hand with the new administration to advance an agenda to improve the lives of the American people," Mr Ryan said in a statement, adding that Mr Trump's victory "marks a repudiation of the status quo of failed liberal progressive policies".

Read more: US Election - Donald Trump vows to be a 'president for all Americans' after shock victory

Both parties' candidates and outside groups spent nearly 1.1 billion dollars combined on House campaigns, short of the 1.2 billion dollar record in 2012, according to the Centre for Responsive Politics, a non-partisan research group.

Republicans had only a slight financial edge.

While Mr Trump hurt Republicans in some areas, his appeal to working-class white voters and their antipathy to Mrs Clinton helped candidates in others.

That seemed to help Republicans limit an erosion of their House majority, which would have left hardline conservatives with added clout to frustrate party leaders. A major loss of moderate Republicans would have increased dissident Republicans' leverage next year.