TAMPA, Fla. -- Even as his path to the presidency narrows, a defiant Donald Trump is insisting he is "winning" and urging his supporters to defy what he is calling an establishment conspiracy to deny the White House to his populist movement.

Trump, in the middle of a three-day swing through battleground Florida as thousands began voting there in person, hammered the "disgusting" media on Monday for its "phoney polls" that he claimed were the latest signs of a "rigged election."

"The media isn't just against me. They're against all of you," Trump told cheering supporters in St. Augustine. "They're against what we represent."

"I believe we're actually winning," he said.

But even as Trump publicly displayed his trademark bravado, his team conceded publicly as well as privately that he was trailing -- and that crucial Pennsylvania may be slipping away to Democrat Hillary Clinton. That would leave him only a razor-thin pathway to the 270 electoral votes needed to win the White House on Nov. 8.

In more bad news for Trump, a new poll shows young voters turning to Clinton now that the race has settled down to two main candidates. Clinton now leads among likely voters 18 to 30 years in age by 60 per cent to 19 per cent, according to a new GenForward survey.

Young black voters already were solidly in her corner, and now young whites are moving her way, according to the survey by the Black Youth Project at the University of Chicago with the Associated Press-NORC Center for Public Affairs Research.

With Trump on the defensive, Clinton worked to slam the door on his candidacy in swing state New Hampshire while eyeing a possible Democratic majority in the Senate.

The former secretary of state campaigned alongside New Hampshire Gov. Maggie Hassan, who is running for the Senate, and Massachusetts Sen. Elizabeth Warren, who was merciless as she seized on recent revelations of Trump's predatory sexual language and several allegations of sexual assault.

"He thinks that because he has a mouth full of Tic Tacs, he can force himself on any woman within groping distance," Warren charged. "I've got news for you Donald: Women have had it with guys like you."

Trump has denied all the recent allegations, and he addressed a new one Monday in an interview with WGIR radio in New Hampshire.

He called the accusations "total fiction" and lashed out at former adult film performer Jessica Drake, who said Saturday that he had grabbed and kissed her without permission and offered her money to visit his hotel room a decade ago.

"One said, 'He grabbed me on the arm.' And she's a porn star," Trump said. He added, "Oh, I'm sure she's never been grabbed before."

With Election Day two weeks away, Trump's electoral map looks bleak.

The Republican National Committee ignored him altogether in mailers to New Hampshire voters set to be distributed later this week, according to material obtained by The Associated Press. The mail focuses instead on Clinton's credibility, featuring a picture of her and former President Bill Clinton and the words: "No More of The Lying Clintons."

Trump's campaign manager Kellyanne Conway outlined a path to 270 electoral votes on Sunday that banks on victories in Florida, Ohio, Iowa and North Carolina along with New Hampshire and Maine's 2nd Congressional District. Assuming Trump wins all of those -- and he currently trails in some -- he would earn the exact number of electoral votes needed to win the presidency and no more.

Noticeably absent from the list was Pennsylvania, a state that a top adviser privately conceded was slipping away despite Trump's aggressive courtship of the state's white working-class voters. The adviser spoke on the condition of anonymity to share internal discussions.

Florida was largely the focus on Monday as in-person early voting began across 50 counties, including the state's largest.

Democrats would take the Senate majority if they pick up four seats and Clinton wins the White House.

Peoples reported from Washington. AP writers Ken Thomas in Manchester, New Hampshire, Kathleen Ronayne in Miami, Tom Beaumont in Des Moines, Iowa, Nicholas Riccardi in Denver and Laurie Kellman and Emily Swanson in Washington contributed