A new poll showing Hillary Clinton’s lead over Donald Trump is growing is evidence of just how difficult a few weeks it’s been for the Republican presidential nominee, says a professor at Princeton University.

A poll for ABC News shows Clinton is eight points ahead of Trump, 50 per cent to 42 per cent among registered voters. Almost six in 10 voters say Trump is not qualified to be president and just three in 10 say they would feel comfortable if he were to become president.

The poll also shows strong disapproval for Trump’s treatment of the Muslim family of a fallen American soldier who spoke out against him at the Democratic National Convention.

A similar poll in mid-July, before either party held its national convention, showed just a four-point advantage for Clinton.

“(Trump) has had a very difficult few weeks. And I think even though there is some discussion he’s reversed his fortunes, the polls are not very good for him right now,” Julian Zelizer, who teaches history and public affairs at Princeton, told CTV News Channel Sunday.

Clinton’s overall lead is growing, Trump’s performance in key states is weak and his “vulnerabilities” in temperament and policy are “very problematic,” said Zelizer.

Trump’s attack on the parents of Army Capt. Humayun Khan, who was killed in Iraq in 2004, has been especially damaging among veterans in swing states, said the professor.

“So he has a lot to overcome if he’s going to mount a serious campaign in the coming months.”

Zelizer says that will mean putting together a three-week period that is free of missteps that raise concerns among the electorate, including when he speaks on Monday at the Detroit Economic Club.

Zelizer also says President Barack Obama is playing a prominent role in the campaign for a number of reasons. First, Obama enjoys very high approval ratings for a president in the last part of his second term, so he’s an asset for the Democrats.

Second, Obama feels a “genuine threat” to what he’s accomplished if Trump takes the White House.

“And finally, I think it’s personal. I think he really believes that Donald Trump is appealing to some of the worst elements in the electorate, which are antithetical to everything he’s tried to work on, to who he is as a politician. So I think all of that is leading him to be quite aggressive in stating what he thinks about this nominee.”