The surveys were conducted between August 5 and 9, with at least 600 voters in each state.
Kamala Harris now leads Donald Trump in three crucial battleground states, according to new polls published Saturday, apparently eroding the advantage the former president has enjoyed there over the past year.
The polls of likely voters by The New York Times and Siena College showed Harris leading Trump by an identical 50 percent to 46 per cent margin in Michigan, Pennsylvania and Wisconsin.
Under the US Electoral College voting system, those three populous Midwestern states are considered key to victory for either party.
The polls are a reversal of surveys in those states which for nearly a year had shown Trump either tied with or slightly leading Democratic President Joe Biden, who dropped out of the race last month and endorsed Harris instead.
Much can change in the nearly three months before the November 5 election. The polling showed voters still prefer Trump on the key issues of the economy and immigration, though Harris had a 24-point advantage when voters were asked whom they trust on the issue of abortion.
Democrats, in any case, have taken heart in the surge of enthusiasm that has greeted Harris's candidacy, with many expressing relief after 81-year-old Biden stepped aside.
Her announcement Tuesday of Minnesota Governor Tim Walz as her running mate also appears to have energized Democrats.
The Harris-Walz surge helped cut short a rise in support for Trump that followed the July 13 attempt on his life and the Republicans' successful national convention last month.
But Harris has enjoyed an even bigger bump in favorability -- up 10 points among registered voters in Pennsylvania in just a month, the Times/Siena polling found.
Voters said they saw her as more intelligent than Trump and having a better temperament to govern.
Trump, his running mate J.D. Vance and other Republicans have tried an array of attacks meant to weaken Harris -- with Trump even questioning her racial identity.
But the new polls show Democrats strongly supporting the younger and more vigorous Harris who, with Walz, has been campaigning at a furious pace this week in swing states.
Among Democrats, voter satisfaction with their choice of candidates has shot up by 27 points in the three Midwestern states since May, the polls found.
Three months ago, it was Republicans who expressed a higher level of satisfaction.
The surveys were conducted between August 5 and 9, with at least 600 voters in each state.
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