President Barack Obama has spoken up for Democratic presidential nominee Hillary Clinton in a fundraising speech, suggesting her campaign is being hurt by sexism in America.

"There's a reason why we haven't had a woman president, that we as a society still grapple with what it means to see powerful women," Obama said at a Democratic Party fundraiser in New York City on Sunday (18 September).

"It still troubles us in a lot of ways, unfairly, and that expresses itself in all sorts of ways."

He also used the opportunity to rip into Republican candidate Donald Trump, CNN reported.

"It's an infomercial, it's a reality show," he said, characterising Trump's campaign presentations. "This guy is not qualified to be president."

Political skirmishes continued elsewhere as well. Trump supporters spent Sunday blaming Clinton for beginning the so-called "birther" rumour that Obama was born outside the US — when in fact Trump has been a key proponent of the tale.

Trump, meanwhile, was attacked by Democratic vice presidential candidate Tim Kaine for "inciting" violence against Clinton by challenging her Secret Service guards to not carry guns. "Take their guns away, she doesn't want guns. Take them, let's see what happens to her," Trump said provocatively during a recent campaign speech.

Trump's GOP vice presidential candidate Mike Pence defended the comments by saying Democrats shouldn't expect to be protected by gun-carrying law enforcement if they want stricter gun control. In fact, Democrats have never argued to disarm law enforcement.

As for Trump, NBC reported he attacked former Defence Secretary Robert Gates as a "clown" and a "mess" for questioning in the Wall Street Journal Trump's readiness to be the commander in chief.

The gun quip and Trump's statement that Clinton started the birther movement were both cited in a New York Times article about the Republican candidate setting a "new low" in politics.

"It's frightening," Vin Weber, a former Republican congressman from Minnesota, told the New York Times. "Our politics, because of him, is descending to the level of a third-world country. There's just nothing beneath him. And I don't know why we would think he would change if he became president. That's what's really scary."

But according to polls, Trump's supporters see him as more likely to aggressively confront what they see as a corrupt political system, even if they recognise him as a risky choice.