With only three months left to go until November's US presidential election, an increasing number of Republicans are speaking out against the party's candidate Donald Trump.

Although he officially accepted the party's nomination at last month's Republican National Convention and - publicly, at least - has the support of the GOP committee, various officials have broken ranks to voice their opposition to Trump.

So who exactly are the dissenters, and what have they to say? 

Susan Collins

Susan Collins has served as a senator for Maine since 1997. In a Washington Post op-ed published last night, she explained that she will not be voting for Trump.

"This is not a decision I make lightly, for I am a lifelong Republican," she explains. "But Donald Trump does not reflect historical Republican values nor the inclusive approach to governing that is critical to healing the divisions in our country."

Collins cites three reasons that helped her come to the conclusion that Trump "lacks the temperament, self-discipline and judgment required to be president":

• An incident in which he mocked a disabled reporter
• His claim that a judge could not rule fairly in a case involving Trump University because of his Mexican heritage
• His criticism of the parents of Humayun Khan, a US army officer killed in Iraq

"The unpleasant reality that I have had to accept is that there will be no 'new' Donald Trump, just the same candidate who will slash and burn and trample anything and anyone he perceives as being in his way or an easy scapegoat," she adds.

Richard Hanna and Adam Kinzinger

Two different US congressmen went public with their criticisms of Trump last week in the aftermath of the national convention.

New York politician Richard Hanna wrote an op-ed for the Syracuse website, saying that he will vote for Hillary Clinton despite disagreeing with her on "many issues."

He described Trump as a man "deeply flawed in endless ways."

However, we went further and also had some harsh words about his party's electoral process, saying he is increasingly of the view that they cannot find a candidate who is "electable as president."

"The primary process is so geared toward the party's political base, which ignores the fact that we have largely alienated women, Hispanics, the LGBT community, young voters and many others in general," he observed.

Meanwhile, Illinois congressman Adam Kinzinger told CNN he also can't support Trump.

"He's crossed so many red lines that a commander in chief or a candidate for commander in chief should never cross," he argued.

The above names are not the only senators and congresspeople speaking out against Trump, and there could well be more critics to come. The Washington Post rounds up some of the other 'dissenters' and their various motivations - from ambitious young politicians looking to bolster their own career, to retiring politicans with "nothing to lose."

For others, criticising Trump is not just a recent development. Check out this open letter to Trump supporters published by senator Ben Sasse in February.

50 Republican national security experts

One of the biggest news stories about the election earlier today was the release of an open letter, signed by 50 Republican national security and foreign policy experts, claiming Trump "would be the most reckless president in American history."

In the strongly-worded letter, the signatories write: "From a foreign policy perspective, Donald Trump is not qualified to be President and Commander-in-Chief. Indeed, we are convinced that he would be a dangerous President and would put at risk our country’s national security and well-being."

They also suggest: "Mr Trump has demonstrated repeatedly that he has little understanding of America’s vital national interests, its complex diplomatic challenges, its indispensable alliances, and the democratic values on which US foreign policy must be based. At the same time, he persistently compliments our adversaries and threatens our allies and friends."

You can read the full letter here, with signatories including former officials with the US department of defence and White House national security council from across various administrations.

Trump himself released a statement responding to the letter, saying: "The names on this letter are the ones the American people should look to for answers on why the world is a mess, and we thank them for coming forward so everyone in the country knows who deserves the blame for making the world such a dangerous place. They are nothing more than the failed Washington elite looking to hold onto their power."

Trump's former rivals

Some of the other Republican presidential hopefuls were quick to endorse Trump when his primary dominance became apparent - Ben Carson, for example, barely waited a week after the end of his own campaign before endorsing the businessman.

Congratulations to @realDonaldTrump. The people have spoken, and it is now time to unite and focus on defeating Hillary Clinton.

— Dr. Ben Carson (@RealBenCarson) May 4, 2016

Others, however, have been far more reluctant to support Trump - and in some cases actively hostile to the prospect.

Ted Cruz made headlines after he failed to endorse the party candidate during a speech at the Republican National Convention - only mentioning Trump's name once.

John Kasich has so far declined to endorse Trump, telling CNN "there's so much water over the dam now, it's become increasingly difficult." He has not ruled out the possibility, however.

Jeb Bush, on the other hand, has insisted he will not back Trump.

"Donald Trump has not demonstrated that temperament or strength of character. He has not displayed a respect for the Constitution. And, he is not a consistent conservative," he argued.

Evan McMullin 

Evan McMullin is not exactly a high-profile figure - well, until yesterday.

Despite being a former CIA operative and chief policy director for the US House of Representatives' Republican Conference, he was far from a household name.

However, he has now been put forward as an independent alternative conservative candidate for Republicans disillusioned with the official candidate.

On his website, McMullin argues: "Donald Trump appeals to the worst fears of Americans at a time when we need unity, not division. Republicans are deeply divided by a man who is perilously close to gaining the most powerful position in the world, and many rightly see him as a real threat to our Republic.

"Given his obvious personal instability, putting him in command of our military and nuclear arsenal would be deeply irresponsible. His infatuation with strongmen and demagogues like Vladimir Putin is anathema to American values. We cannot and must not elect him."

Reuters suggests there is "virtually no chance" McMullin can win, and may not even be able to get on the ballot in key states.

However, 'Never Trump' Republicans - not to mention Democrats and others - may well be hoping McMullin's votes, however few, could split the vote enough to deny Trump a victory in a few electoral battlegrounds (all without having to vote for Hillary Clinton).