US Vice President Kamala Harris has promised a "fight for the future" as new opinion polls showed her narrowing the gap with Republican rival Donald Trump.
Harris' swift emergence as the successor to President Joe Biden, 81, as the Democratic presidential candidate in the November 5 election has shaken up a stagnant presidential race, with multiple polls showing her narrowing former president Trump's advantage.
In an address in Houston to the American Federation of Teachers, Harris, 59, focused on economic policy and workers' rights, touting plans for affordable healthcare and child care and criticising Republicans for blocking gun limits in the wake of school shootings.
"Ours is a fight for the future," Harris told a crowd of about 3500 people.
"We are in a fight for our most fundamental freedoms. And to this room of leaders, I say: Bring it on."
A series of polls conducted since Biden ended his re-election bid on Sunday, including one by Reuters/Ipsos, show Harris and Trump beginning their head-to-head contest on roughly equal footing, setting the stage for a close-fought campaign over the next three-and-a-half months.
A New York Times/Siena College national poll published Thursday found Harris has narrowed what had been a sizeable Trump lead. Trump was ahead of Harris 48 per cent to 46 among registered voters, compared with a 49 per cent to 41 in early July, following Biden's disastrous debate performance that led to a wave of Democratic calls for him to step aside as candidate.
While nationwide surveys give important signals of American support for political candidates, a handful of competitive states typically tilt the balance in the US Electoral College, which ultimately decides who wins a presidential election.
Harris also got good news on that front as Emerson College/The Hill published a poll finding that she had begun to close the gap with Trump in five critical battleground states: Arizona, Georgia, Michigan, Pennsylvania and Wisconsin. Trump still narrowly leads Harris in all but Wisconsin, which is tied, according to the poll of registered voters in those states.
Together, the polls suggest that while Trump, 78, retains a narrow advantage, he has not seen the sort of bump in support following last week's Republican National Convention that candidates hope to get out of the highly scripted, televised and expensive events.
Trump on Wednesday night laid into Harris in his first rally since she replaced Biden atop the ticket. He continued his criticism online on Thursday.
"We're not ready for a Marxist President, and Lyin' Kamala Harris is a RADICAL LEFT MARXIST, AND WORSE!" Trump posted on his social media platform.
More than 40 former US Justice Department officials, primarily from Democratic administrations, signed a letter endorsing Harris and calling Trump a threat to the rule of law in the United States.
Former president Barack Obama has been in regular contact with Harris and plans to soon endorse her as the Democratic presidential candidate, a source familiar with his plans said on Thursday.
Former US House of Representatives Speaker Nancy Pelosi, who retains deep influence within the Democratic Party, endorsed Harris on Thursday alongside dozens of other female lawmakers.
The Harris campaign released its first video advertisement online on Thursday. Harris narrates the ad, framing the campaign as a battle to protect Americans' individual liberties to the sound of Beyonce's song "Freedom."
The next highly anticipated development will be Harris' choice of a vice-presidential candidate to counter Trump's selection of US Senator JD Vance of Ohio.
The list of contenders amounts to a who's-who of rising Democrats, including US Senator Mark Kelly of Arizona, Governors Josh Shapiro of Pennsylvania, Roy Cooper of North Carolina and Andy Beshear of Kentucky, as well as Transportation Secretary Pete Buttigieg.
