Donald Trump is struggling to gain the support of women and minorities that he would need to win the U.S. presidential election this fall. But the real reason Trump is trailing Hillary Clinton by such a wide margin is key demographic groups that have traditionally voted for the Republicans are abandoning him.

The last time the GOP won the White House, in 2004, George W. Bush won the votes of male, white and wealthier Americans by double-digit margins. But this year, the Republican nominee is down significantly among these demographics — even by the bars set by Mitt Romney in his failed 2012 bid for the presidency.

But while Hillary Clinton has held on to most of Barack Obama's coalition of voters from 2012, she hasn't built on it. Nevertheless, due to the significant losses that Trump has suffered among groups that should be in the middle of the Republican tent, she has moved ahead in the national polls by about six points while also being on track to take less of the vote than her Democratic predecessor did in either of the past two elections.

Unlike in Canada, exit polls are conducted on election day in the United States, making it possible to get a very detailed indication of how Americans voted according to their demographic profile. By comparing these numbers to an average of polls conducted in August that have included demographic breakdowns, we can see where Trump and Clinton stand relative to previous elections.

And the numbers are bad for Donald Trump.

Clinton's big lead among women

Men have traditionally been more likely than women to vote for Republican presidential candidates, and that remains the case in this campaign. Trump trails Clinton by just one point among men. But he trails Clinton by 15 points among women.

Males were an electorate Romney won easily in 2012, beating Obama by a margin of seven points.

Margins between the two U.S. presidential candidates in 2016 and 2012, based on an average of recent polls (2016) and exit polls (2012). Among women, Clinton's advantage of 15 points is bigger than Obama's was in both 2008 and 2012.

But Clinton hasn't gained new supporters. Instead, she is down about two points from Obama's 2012 results among both decided male and female voters. The difference from four years ago is that Trump is down eight points among men and 10 points among women.

That Clinton can have a larger lead among these groups than Obama did in 2012 while losing support is because more voters are backing third party candidates like Gary Johnson of the Libertarians and Jill Stein of the Greens.

But considering Green Party supporters come from the left side of the political spectrum and Johnson appears to be drawing support away from both major candidates in largely equal parts, it doesn't appear likely that Trump would benefit disproportionately from any future reduction in third party support.

Trump way off Bush's mark

The key to Republican victories in the past has been to win by a wide enough margin among whites to make up for the huge Democratic advantage among non-white voters.

But Trump is instead on pace to match the sort of margins put up by failed Republicans in the 1990s. He is leading by an average of five points over Clinton among white Americans, a group that Romney won by 20 points.

Margins between the two U.S. presidential candidates in 2016 and 2012, based on an average of recent polls (2016) and exit polls (2012). In 2004, Bush won the vote among whites by 17 points — the Texan also took 44 per cent of the vote among Hispanics.

Trump is nowhere near that mark, averaging about 19 per cent support among Hispanics. Clinton is winning this group by 35 points. That is less than Obama's 44-point margin in 2012, but this is partly due to the strong support third party candidates are currently getting among Hispanics.

Barack Obama, the country's first black president, won the vote among black Americans by a margin of 87 points in 2012. Even John Kerry, the Democratic nominee who came up short in 2004, won black voters by 77 points.

Clinton's lead among black voters currently stands at about 75 points. Among decided black voters, she is running six points worse than Obama did in 2012, while Trump is on track to replicate Romney's performance of about six per cent.

Clinton gains ground with the wealthy

Romney was estimated to be worth about $250 million during the 2012 campaign and won the vote of families with a household income of $100,000 or more by 10 points. Donald Trump, who is estimated to be worth a few billion dollars, is losing the vote of this group of wealthier Americans to Hillary Clinton by seven points.

Margins between the two U.S. presidential candidates in 2016 and 2012, based on an average of recent polls (2016) and exit polls (2012). This is one slice of the electorate in which Clinton is on track to make gains. She is up about four points over Obama's performance among this group in 2012, while Trump is down 14 points.

This also extends to the middle class. Among households earning $50,000 and $100,000, Clinton is ahead by four points. Romney won this demographic by six points.

But among the poorest Americans, Clinton is on track to do worse than her predecessor. She leads among voters in households earning $50,000 or less by 13 points, compared to a 22-point victory by Obama in 2012. And while Clinton is down nine points among decided poorer voters compared to 2012, Trump is down just two points.

But one of the biggest shifts — partially related to household income — has been among college graduates. Romney won these voters by four points in 2012. Clinton is on pace to win this electorate by 22 points.

Donald Trump may "love the poorly educated," but he will also need to win back the support of other groups that have traditionally supported the Republicans in the past — not only to give himself a chance of being in the White House next year.

He needs those formerly reliable Republicans on his side if he is to avoid a landslide defeat in November.