There is a growing movement among US Republicans to prevent Donald Trump becoming the party's nominee for president.

The discontent of several top members of the party has followed the controversial billionaire since he announced his candidacy a year ago. But now, four weeks before the party convention, their anti-Trump sentiments have reached new heights.

Trump emerged from state-by-state voting that began in February as the strongest candidate by far. He secured a majority of delegates, making him the presumptive nominee when the party convenes in Cleveland next month.

But the desire of some to dump Trump has never completely gone away.

Criticism of Trump - a real estate tycoon and political maverick - among party members comes from different directions.

First is his campaign against a judge who is presiding over a trial against him involving Trump University. The judge is a US citizen born to parents originally from Mexico. Trump says the judge may be biased against him because he wants to build a wall along the US-Mexico border.

The attempt to meddle in a court case has been described by leading Republicans as racially motivated and fundamentally wrong.

The second reason is Trump's response to Sunday's mass shooting in Orlando, Florida, in which 49 people were killed and 53 were injured. Many considered both the content and his tone to be undignified.

Third is his inability to unite the party since his victory in the primaries. Recent public opinion polls show a growing distance between Trump and the presumptive nominee of the Democratic Party, Hillary Clinton.

An average of eight polls compiled by the website shows Clinton has the support of 44 per cent to 38 per cent for Trump.

Finally, the way Clinton's campaign has started to catch on also has been a reason behind the criticism.

One effort to dump Trump has been mouned by several dozen delegates to the convention and is the most organized attempt of its kind, according to the Washington Post.

"This literally is an 'Anybody but Trump' movement," said Kendal Unruh, a Republican delegate from Colorado who is leading the campaign. “Nobody has any idea who is going to step in and be the nominee, but we’re not worried about that. We’re just doing that job to make sure that he’s not the face of our party.”

The party leadership has been taking a strikingly restrained position to growing doubts and public bickering. The powerful speaker of the House of Representatives, Paul Ryan, officially supports Trump but also has remained ambiguous.

In an interview with US television network NBC, Ryan said Republicans should follow their consciences on whether to support Trump.

"The last thing I would do is tell anybody to do something that's contrary to their conscience," Ryan said, according to an excerpt of of the interview released by the network. "I get that this a very strange situation. [Trump is] a very unique nominee."

However, Ryan added that he felt a responsibility not to lead a "chasm" within the party.

But it was the "conscience" comment that was left open to interpretation. It was seen as tacitly endorsing an "out" clause ahead of the convention: that delegates obligated to support Trump should be exempted from having to do so when the formal nomination vote takes place. They should be able to vote for whomever they choose at the convention.

Trump responded to the rumblings against him by saying the nearly 14 million votes he received in the primaries were far more than any other candidate had received in the history of the Republican primaries.

"I have tremendous support and get the biggest crowds by far," he told the Washington Post.

He said any move encouraging delegates to abandon their obligation to support him "would not only be totally illegal but also a rebuke of the millions of people who feel so strongly about what I am saying."

Previous attempts by Republicans to stop Trump were either uncoordinated, too timid or soon collapsed. This one may be the same, or it may not matter. Trump already has said if the Republicans don't support him, he could go his way alone.

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