Democratic presidential hopeful Hillary Clinton is closing fast on Bernie Sanders in New Hampshire, according to two new polls, and Marco Rubio has overtaken Ted Cruz for second place among Republicans.

An NBC News/WSJ/Marist poll released Thursday afternoon, just five hours before a critical debate in the college town of Durham, puts Clinton 20 points behind Sanders – a sign that her comeback may have begun just in time.

Bernie, a Vermont senator known for his democratic socialist politics, has the support of 58 per cent of Granite State Democrats in that poll, and posted the same number in a UMass Lowell / 7News overnight tracking poll earlier in the day.

Clinton has 36 per cent in the UMass poll and 38 per cent in the NBC News survey. That latter number would put her just 20 points back as Tuesday's New Hampshire primary draws near.

For comparison, she trailed Sanders by 29 points just one day ago in the UMass Lowell poll.

EPIC COMEBACK? Hillary Clinton has closed a one mammoth gap with Bernie Sanders, posting a more modest 20-point deficit just a few days before the New Hampshire primary

SHRINKING: Bernie Sanders' lead over Hillary Clinton is getting smaller, according to the latest UMass tracking poll, although it's not as small as NBC's gap

RUBIO RISING: Marco Rubio has passed Ted Cruz for second place among Republicans in New Hampshire

RACING AHEAD: Marco Rubio (right) is now running in second place among Republicans in the New Hampshire prmiary, according to a three-day tracking poll

That poll samples 1,400 New Hampshire residents every night and reports three-day rolling averages.

On Wednesday Sanders led Clinton by a 61 per cent to 32 per cent margin, according to the UMass pollsters. A day later, that lead has shrunk to 58–36.

That suggests a huge 'bounce' from Clinton's razor-thin victory in the Iowa caucuses, since the change came from polling conducted after that contest's photo finish was finally sorted out.

Tracking polls typically show small shifts of a point or two from day to day.

Billionaire front-runner Donald Trump, for instance, now leads his nearest opponent Marco Rubio by 21 points among likely Republican primary voters. His lead was 23 points a day ago.

Rubio, a Florida senator and surprisingly strong third-place finisher in Iowa, overtook Texas Sen. Ted Cruz to finish in second place in the latest UMass poll.

Trump sits at 36 per cent; Rubio at 15; and Cruz at 14.

STILL THE ONE: Trump seems unstoppable in New Hampshire with a strong double-digit lead

No one else in the Republican field posted a double-digit number. Jeb Bush, the former FLorida governor whose moderate politics should play well in New Hampshire, polled only 8 per cent. Ohio Gov. John Kasich and New Jersey Gov. Chris Christie, both swimming in the same lane as Bush, are at 7 per cent and 5 per cent respectively.

The more conservative retired surgeon Ben Carson and former CEO Carly Fiorina are languishing with just 4 and 3 per cent.

The New Hampshire primary elections will be held on Tuesday.