Donald Trump has opened up a sizable lead over Texas Sen. Ted Cruz in a new national poll of Republican voters, widening his margin from 3 percentage points to 18 in the space of three weeks.

The Fox News poll found 45 per cent of GOP voters support his upstart bid for the White House, while Cruz has the backing of 27 per cent.

John Kasich, the long-shot Ohio governor, remains in third place with 25 per cent.

Trump's 45 per cent showing is a high water mark for him in the Fox poll, although he has posted number as high as 53 per cent in other recognized surveys.

Three weeks ago the 45-27 margin between the real estate billionaire and the tea party senator was just 41-38, with Kasich far behind at 17.

WIDER: Billionaire Donald Trump has grown his nationwide lead over Texas Sen. Ted Cruz in the Republican presidential primary fight from 3 points to 18 in just the past three weeks

FINISH LINE IN SIGHT? Trump leads Cruz in delegates and votes, with 1.94 million more Americans choosing him at the polls

THEY THINK THEY CAN (AND AMERICANS SAY ONE OF THEM IS RIGHT): Ted Cruz (left) and John Kasich (right) are chugging along, and polls show Kasich is best equipped to beat Hillary Clinton

Fox News reported that Trump's most loyal demographic continues to be Republicans without college degrees. Fully 54 percent of them are on the Trump train.

Separately, 50 per cent of Republicans who call themselves 'very' conservative are in his corner.

That outcome continues to surprise some analysts who see Cruz as the most likely mascot for the far right. But Trump has defied expectations since he launched his unconventional White House bid last June.

Fox also polled a handful of hypothetical matchups between Republican and Democratic candidates, and found that it's Kasich who performs best – even though he has the slimmest chance of winning the GOP nomination.

Kasich would beat former Secretary of State Hillary Clinton by nine points if they squared off today, pollsters found.

Clinton would top Trump by 7 points, and Cruz by 1 point.

The Ohio governor is also best-positioned to beat Vermont Sen. Bernie Sanders, who is running in second place on the Democratic side of the race.

Kasich would edge Sanders by 4 points if they were the two main White House candidates, the poll determined. Trump and Cruz would drown against Sanders, but 14 and 12 points respectively.

Yet voters still say they believe Trump is the Republican with the best chance of beating Clinton in the fall. Forty-two per cent pick him, followed by Cruz with 24 per cent and Kasich with 20.

Among Republican primary voters only, that gap grows. Fifty-eight per cent of the GOP faithful give Trump the best odds of defeating Clinton, followed by 21 per cent for Cruz and 16 per cent for Kasich.

TIGHTENING: Bernie Sanders has drawn to within 2 points of Hillary Clinton on the strength of an 8-and-1 streak of primary and caucus victories

'When it comes to electability, the disconnect between what Republicans think and what the polling data show is astounding,' said Daron Shaw, the Republican half of the bipartisan team that conducts the Fox news poll.

'Close to three times as many think Trump has the best chance against the Democrats despite the fact he's down seven points to Clinton while Kasich is up nine.'

The Fox poll showed the Democratic primary race tightening even as Trump enlarged his lead on the GOP side.

Former Clinton now leads Sanders by just 2 points, with a 48-46 per cent margin.

Sanders has won eight of the last nine Democratic primary contests and caucuses in the last month. Before that streak began last month, he was trailing by 13 points.

Much of that shift, Fox reported, is a reflection of changing views among Democratic women.

Since mid-March, Clinton has lost 11 percentage points among women in her primary contest, and Sanders picked up 9 percentage points in from the same group.