Republican Party leaders might be preparing to turn on Donald Trump again, after America’s most historically reliable polling companies show the billionaire’s bid for the presidency is losing momentum.

The Speaker of the US House of Representatives, Paul Ryan, has cautiously reopened the prospect of a delegates revolt against Mr Trump at the Republican Convention in July.

In a television interview on Sunday, Mr Ryan refused to rule out the possibility that delegates might turn on Mr Trump even though many of them were elected on the basis they would support the New York tycoon.

• We need more guns: Trump
• Trump un-American: Obama unleashes hell
• Trump calls for Obama resignation after shooting

“They [the delegates] write the rules – they make the decisions,” Mr Ryan told NBC. “All I want to make sure is that it is done above board, clearly, honestly and by the rules.”

Later in the interview, he said the “last thing” he would ever do is “tell anybody to do something that’s contrary to their conscience”.

The renewed speculation that Mr Trump may be dumped as the Republican nominee comes as a small group of delegates campaign to change the rules so that delegates bound to vote for Mr Trump can vote for another candidate.

Polls show Clinton’s lead over Trump increasing. Photo: AAP

“About the most un-Republican thing the party of [Abraham] Lincoln and [Ronald] Reagan can do is compelling its members to violate their own conscience,” a statement being circulated by the group reads.

It also follows the publication of a string of opinion polls this month indicating he is headed for defeat against Democratic presumptive nominee Hillary Clinton.

Perhaps the most significant poll was the IBD/TIPP survey released on June 6 before the Orlando gun tragedy. It showed Mr Trump trailing Mrs Clinton by 45 per cent to 40 per cent.

The IBD/TIPP poll is renowned for being one of America’s most accurate surveys of voting intentions after coming within 0.1 per cent of predicting the outcome of the 2012 presidential election.

Other polls indicate Mr Trump’s popularity has taken a nosedive since the Orlando shooting, with many voters unimpressed by public comments he made after 49 people were killed.

Polls taken by Reuters and CBS since the tragedy show Mrs Clinton’s lead may have increased to as much as 10 percentage points.

And Mrs Clinton now has clear leads in key swing states that will decide the November election, including Florida, Virginia and Ohio.

‘Insecure money grubber’

Democrat politicians have stepped up their attacks on Mr Trump in the past week as public opinion turns on the embattled presumptive Republican nominee.

Outspoken Massachusetts senator Elizabeth Warren ripped into Mr Trump last weekend describing him as a “money grubber” and “thin-skinned racist bully”.

Democrats Senator Elizabeth Warren has been an outspoken Trump critic. Photo: Getty

“Every day we learn more about him, and every day it becomes clearer that he is just a small, insecure money grubber who doesn’t care about anyone or anything that doesn’t have the Trump name splashed all over it,” she told a meeting of Democrat supporters on Saturday.

Senator Warren also depicted Mr Trump as a business failure, trumpeting a long list of his commercial ventures that left consumers out of pocket, including the controversial Trump University education business.

“Now Trump University failed, and that’s no surprise,” Senator Warren said. “Think about all the other Trump failures: Trump casinos, Trump Airlines, Trump steaks, Trump magazines, Trump vodka, Trump Mortgage, Trump Games, Trump Travel, Trump Ice, Trump Network.

“Donald Trump is a proven businessman – a proven failure.”

US political experts view Senator Warren as one of Mrs Clinton’s leading prospects for a vice-presidential running mate.