AFP/ Washington

Frontrunner Hillary Clinton has extended her lead over fellow White House hopeful Bernie Sanders in the 2016 Democratic nomination race, while her leading Republican rival Donald Trump has done the same, polls showed Tuesday.

Clinton impressed in her party's first televised debate last week in Las Vegas, while Trump keeps rising as the billionaire real estate tycoon taps into nationwide anti-Washington sentiment.

Vice President Joe Biden, the center of feverish speculation over his possible entry in the fight to be the next US commander in chief, sits third in a NBC/Wall Street Journal poll on the Democratic race.

But it is Clinton who is firmly in the lead, 20 points ahead of Sanders, with Clinton getting the backing of 49 percent of respondents, up from 42 percent last month.

Support for Sanders, an independent senator from Vermont and the only genuine challenger to ex-secretary of state Clinton, is down from 35 percent to 29 percent.

The poll offers scant encouragement for Biden, with support marginally down at 15 percent.

The survey, which included 400 people who said they would vote in a Democratic primary, was conducted October 15-18.

Clinton's performance in the October 13 debate was widely seen as giving her campaign renewed momentum.

She cut a calm and confident figure as she parried jabs by Sanders and three other hopefuls, who challenged her on everything from political U-turns to gun control and military intervention in the Middle East.

Meanwhile, Trump's performance has only improved, as his support reached 27 percent in a new CNN/ORC poll of Republicans and Republican-leaning independents. That is up three points from a month earlier.

But the biggest gain was enjoyed by Ben Carson who has been in second place for nearly two months, with the poll showing the retired neurosurgeon at 22 percent, up eight points since September.

No other candidate came close in the broad Republican field.

Former Florida governor Jeb Bush and Senator Marco Rubio were tied for third at eight percent, while Mike Huckabee and Rand Paul each had five percent.

The candidate taking the biggest hit was former Hewlett-Packard chief executive Carly Fiorina, whose star rose rapidly after a strong debate performance early last month.

But she lost her post-debate bump, shedding 11 points since mid-September to sit at four percent today.

Fiorina, Trump and Carson are political outsiders who have never held public office.

As for why Trump and Carson remain on top: "We've hit a chord," Trump told CNN on Tuesday.

He went so far as to suggest a Trump-Carson outsider ticket was not out of the question.

"I like him, he likes me. I mean, stranger things have happened," Trump said. "But it's too early to think about."