As endorsements go, it was not particularly flattering.

Robert de Niro, star of the 1976 movie Taxi Driver, has said Donald Trump reminds him of the iconic film’s mentally disturbed main character, Travis Bickle. “God help us,” De Niro said on Saturday at the 22nd Sarajevo Film Festival, when asked what he thought of Mr Trump’s candidacy.

Yet the actor is just one of many to have spoken out after a week in which the Republican candidate’s campaign lurched into even more uncharted territory and sought to even question the very legitimacy of the election itself.

De Niro said: ‘Donald Trump is totally nuts’ at the Sarajevo Film Festival (AP)

The week began with Mr Trump suggesting that people could shoot his rival Hillary Clinton (something that earned his campaign a visit from the Secret Service), saw him claim President Barack Obama was the founder of Isis, and concluded with him speaking in Pennsylvania where he said the only way he could lose the fiercely-contested state was if the Democrats cheated.

“We’re going to watch Pennsylvania. Go down to certain areas and watch and study and make sure other people don’t come in and vote five times,” he said at a rally in Altoona.

“If you do that, we’re not going to lose. The only way we can lose, in my opinion - I really mean this, Pennsylvania - is if cheating goes on.”

The comments have created fresh concern among some about the direction Mr Trump’s campaign is taking, as did his announcement that he wants to recruit election observers to ensure the process is not rigged. Politico said the New York tycoon’s move was unprecedented in a US election.

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Mr Trump also took another chance to attack the media, something he has done so in vitriolic terms. “I tell you, the lowest,” he said. “They are the lowest form of humanity.”

The comments led Mr Obama’s former campaign manager, David Axelrod, to comment: “Basic rule of presidential politics: The candidate who blames his problems on the news media is generally losing.”

An average of national polls collated by Real Clear Politics places Ms Clinton anywhere up to ten points ahead Mr Trump. Pennsylvania is always one of the battleground states in a general election, and it will be where he hopes his appeal to working class white voters could defeat his Democratic rival.

The Clinton campaign speculates why Trump has not released his taxes

Ms Clinton’s lead in the polls in the state is currently more than solid. A Quinnipiac survey of likely voters released this week found her leading 52 to 42. Both campaigns are pouring considerable resources into the state.

Yet Mr Trump has insisted he can win there, as long as the election is fair.

“She can’t beat what’s happening here. The only way they can beat it in my opinion, and I mean this 100 percent, if in certain sections of the state they cheat,” he said.

In comments that echoed, and went further than, his statements last week in Ohio that he’s “afraid the election is going to be rigged”, he claimed that Republican leaders in the state were “very concerned” about the possibility of cheating.,

“We have to call up law enforcement, and we have to have the sheriffs and the police chiefs and everybody watching,” he said.

This is not the first time that Mr Trump has raised the issue of potential for fraud in Pennsylvania, one of 20 states that has no ID requirement for polling places and which Mr Obama won in 2008 and 2012. He said Mr Obama’s result was proof of fraud, something that sparked backlash on social media from election inspectors involved in the process who dismissed the tycoon’s claims.

De Niro starred as a mentally disturbed taxi driver in the 1976 movie (Wikipedia)

A comprehensive study of in-person voter fraud between 2000-2014, conducted by Loyola Law School professor Justin Levitt, found 31 incidents nationally out of more than 1 billion ballots cast.

Yet on Saturday, Mr Trump’s spokesperson insisted the concerns were genuine.

“There is no evidence because the election hasn’t occurred yet,” spokeswoman Katrina Pierson told CNN. “This is not far-fetched. Election fraud has been a concern for a very long time.”

According to the Associated Press, in Sarajevo, De Niro could only shake his head about the way the way the media had treated the New York magnate’s campaign.