Republican donors shocked by the rise of Donald Trump have launched a scathing attack ad, which compares the billionaire businessman to Adolf Hitler. Trump, 69, currently leads most polls for the Republican nomination to stand in the presidential elections in 2016.

His campaign has been noted for the controversial things Trump has said, including rounding up illegal migrants and deporting them, making it compulsory that Muslims should be registered with the state, and lauding his supporters as they allegedly beat up a black protestor. However, Trump received the most criticism after he seemingly mimicked a reporter with a disability during a speech.

In the advertisement, Colonel Thomas Moe, a former prisoner of war in Vietnam, paraphrases First They Came For… – a famous poem based on a speech given by Martin Niemöller, a prominent German Protestant pastor, which condemned his fellow German citizens for not speaking out against Hitler in 1946.

"You might not care if Donald Trump says Muslims should register with their ­government, because you're not one," says Moe over ominous music, before going on to list other groups targeted by the former The Apprentice presenter.

The colonel concludes: "But think about this: If he keeps going, and he actually becomes president, he might just get around to you. And you better hope there's someone left to help you."

The recently published advert was produced by the campaign of John Kasich, the Ohio governor and a moderate Republican who is also a challenger to become the Republican leader. Kasich is trailing in the polls with his numbers at 2.8%, according to a poll of polls by the Real Clear Politics website.

Meanwhile, Trump's lead in the polls, which once stood at 30.5% at its peak, has dipped to 28.7%, according the Real Clear Politics, as signs that his abrasive style may be putting off some voters.

Politically incorrect politics

The leading Republican nominee has built his campaign on blurting out un-politically correct comments, which have gone down well with some supporters. "The nastier Donald Trump gets, the more some people like him," commented The Washington Post.

Liz Mair, a vocal supporter of the Republican Party, has also launched an initiative called Trump Card to try to stop the reality TV star.

"This dude is running away with this thing and very few people are doing anything about it," she said. "There are a lot of people who are supporting him who are white supremacist, white nationalist. I don't like that."

Trump, who has branded Mair as a "wacko", has falsely claimed that "thousands" of Muslims in New Jersey celebrated the 9/11 terrorist attacks. He has declined to rule out a national database of Muslims and when asked whether the steps he was advocating were similar to those taken by the Nazis against the Jews, he replied: "You tell me."