US Democratic Party presidential candidate and Vice President Kamala Harris is shifting her campaign strategy to win over more men and Republicans, and sharpening her attacks on Donald Trump as Democratic anxieties rise over polls showing her rival making strong gains, Reuters reports.

The Democratic nominee is in a statistical dead heat in crucial states with Trump, the Republican former president, according to public and internal campaign polls cited by Democratic sources.

It is a frustrating moment for a whirlwind campaign that opened with a huge jump in enthusiasm and cash when she replaced President Joe Biden in July, put on a robust Democratic convention and was widely hailed as the winner of her only debate with Trump.

After weeks of focusing on policy and trying to define herself on the national stage, Harris is going back to her prosecutorial posture of July, an adviser said, hoping to demonstrate strength to voters who might be attracted to Trump's strongman image.

At her Monday night rally in Erie, Pennsylvania, she took the rare step of showing clips of Trump's remarks about an "enemy within" America to underscore her argument that he poses a danger to the country.

Three Trump donors told Reuters in recent days that they were feeling increasingly confident about Trump’s chances, chalking the shift up to Americans' focus on immigration and Trump's flurry of recent rallies and events.

But internal Harris campaign polls show her within the margin of error in each of the seven battleground states that will help determine the winner, two sources with direct knowledge of the matter said.

In the latest batch, she is tied in Pennsylvania, up slightly in Wisconsin, and down in Michigan, one source said.