ATLANTA -- The Democratic National Convention speaker's lineup has highlighted an increasingly diverse country that could soon elect the first female president to succeed its first black chief executive.

Yet the stream of women, African-Americans, Latinos, gay Americans -- from U.S. senators and celebrities to activists and, on Thursday, Democratic nominee Hillary Clinton herself -- also serves as a reminder of Democrats' struggles to connect with most heterosexual white men.

"It's just sad," says Dave "Mudcat" Saunders, a Democratic strategist turned Donald Trump supporter who says his party "has abandoned" culturally conservative white men like himself.

Even Vice-President Joe Biden offered a noteworthy, if more muted assessment Wednesday. Democrats have "done the right thing" for white working-class voters, he said on MSNBC, but the party hasn't "spoken to them."

It's a long-developing gap that bolsters Republican control of Congress and most statehouses. It could play into the hands of Republican Trump, whose path to victory depends on whites drawn to his blistering critiques of elitism and "political correctness" in the America of Clinton and Barack Obama.

White men still make up about a third of the typical presidential electorate and will be crucial to Trump's fortunes in Rust Belt states that have seen a declining middle class. They also could tip the balance in battlegrounds like Virginia and Florida, states Obama won twice.

Saunders says both parties play "wedge and identity politics" on guns, gay rights and other issues.

Republicans emphasized "law and order" at their Cleveland convention, while Democrats on Tuesday welcomed "Mothers of the Movement," black moms whose sons died at the hands of police. Republicans heard National Rifle Association leaders; Democrats are featuring families of gun violence victims.

Democratic pollster Mark Mellman described "a cultural gap," with both parties playing to their advantages. But Saunders says Trump taps into a "legitimate" frustration acute among small-town and rural white men whose fathers and grandfathers once helped elect Democrats.

"They see no opportunity, no hope," continued Saunders, who advised John Edwards' presidential campaigns and Jim Webb's brief 2016 bid. "Then they see Democrats up there talking about diversity and trends and what we'll be like in 40 years. This country needs help now."

Democrats insist they understand the sentiment.

Former President Bill Clinton, speaking Tuesday on his wife's behalf, told of campaigning this year in West Virginia -- a now heavily Republican state that Clinton won twice in the 1990s -- in front of coal miners who blame job losses on Democrats' policies.

"Hillary sent me to tell you that if you really think you can get the economy back you had 50 years ago ... vote for whoever you want to," the 42nd president recalled telling miners. "But if she wins, she is coming back for you to take you along on the ride to America's future."

The white Pittsburgh police chief, Cameron McLay, told Democratic delegates that police and their communities can work together. The top organized labour leader, Richard Trumka, who is white, made the traditional appeal that Democrats are the party of workers.

Clinton's running mate, Virginia Sen. Tim Kaine, will speak Wednesday, and aides note he is well-regarded across his state, including in rural, Republican pockets. Kaine speaks effusively of his family and Catholic faith.

Robby Mook, Clinton's campaign manager, said at a convention lunch sponsored by the Wall Street Journal that Clinton will address working-class issues in her speech.

In 2012, exit polls conducted for the Associated Press and television networks showed Republican Mitt Romney won 62 per cent of white men to Obama's 35 per cent. Measured another way, white men made up less than quarter of Obama's national vote, compared to about 44 per cent of Romney's total.

Current polls suggest a more pronounced gap this year, a trends evident on the ground.

In St. Clairsville, Ohio, not far from where both Clintons campaigned this spring, barber shop owner Kent Jenewein estimated that "more than 90 per cent" of his clients, nearly all white men, back Trump -- in a county Obama won in 2008. Clinton, Jenewein said, "doesn't understand us."

Mellman, the pollster, says Democrats can make "marginal gains" among non-urban whites, including men, in the same way Republicans are seeking marginal upticks in support from African-Americans and Latinos. Trump could also drive up his support among white men only to see Clinton buoyed with better support than Obama had from white Republican women who dislike Trump.

Clinton insisted during the primaries that she won't overlook white men. "Let's be honest," she said in her own Appalachia campaign stop. "Some (of you) find it hard to think about voting for any Democrats. ... But I'm going to keep trying to convince people otherwise."

Associated Press writer Catherine Lucey contributed to this report from Philadelphia.