The Republican Party will retain control of both houses of the US Congress after fighting off attempts by Democrats to wrest control of the upper chamber Senate, the Republicans claimed on Twitter after projected wins in key races early Wednesday.

The victory leaves control of both houses of Congress unchanged, in a win for the centre-right party's legislative agenda.

Democrats had been hoping to wrest power from the Republicans, who, ahead of the elections, had held a 54-44 majority in the upper house, with two independents caucusing with Democrats.

Republicans successfully defended seats that polls had shown in play in North Carolina, Indiana, Wisconsin, Missouri, Pennsylvania, Florida and Arizona, according to US broadcaster projections.

About one-third of the 100 Senate seats and all 435 House seats were at stake.

All signs point to the Republican Party holding on to 18 of their 24 seats that were contested and the party is leading the race for three others.

Projections show the party now has 51 seats in the chamber.

Democrats have secured 11 seats, while in the state of Louisiana, no candidates was able to secure 50 percent of the vote, prompting a run-off election Dec 10.

Democrats now hold 48 seats with Alaska results are expected to be called soon.

As presidential nominee Donald Trump is projected by many to win the White House, Republicans could narrowly secure their lead in the Senate that stood at 54-46 going into the vote.

The Democrats squandered an opportunity to control the Senate as the 34 seats up for grabs were held by Republicans.

In the House of Representative, the picture has not seemed to change.

Surpassing the simple majority of 218, Republicans are currently leading the race 221-160.

Republicans controlled the House with 247 of 435 seats.

Although the congressional picture has not changed significantly, a likely Republican victory for president will have certain political implications in terms of running the government with one party having control of Congress and the White House.

In order for a draft bill to become law, it must be approved by the Senate and the House.

If Trump wins the Oval Office, it remains to be seen how he will work with Republican lawmakers whom he has alienated and to a great extent, who rejected his candidacy.