A new poll showing Hillary Clintonā€™s lead over Donald Trump is growing is evidence of just how difficult a few weeks itā€™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 Trumpā€™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.

ā€œ(Trump) has had a very difficult few weeks. And I think even though there is some discussion heā€™s reversed his fortunes, the polls are not very good for him right now,ā€ Julian Zelizer, who teaches history and public affairs at Princeton, told Ā鶹“«Ć½ Channel Sunday.

Clintonā€™s overall lead is growing, Trumpā€™s performance in key states is weak and his ā€œvulnerabilitiesā€ in temperament and policy are ā€œvery problematic,ā€ said Zelizer.

Trumpā€™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.

ā€œSo he has a lot to overcome if heā€™s going to mount a serious campaign in the coming months.ā€

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 heā€™s an asset for the Democrats.

Second, Obama feels a ā€œgenuine threatā€ to what heā€™s accomplished if Trump takes the White House.

ā€œAnd finally, I think itā€™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 heā€™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.ā€