New York, New York - 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 Democratic presidential candidate Harris leading her Republican rival Trump by an identical 50% to 46% 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.

Harris' apparent lead is within the polls' average margin of error of 4.5 points.

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Nevertheless, the polls show a shift compared to previous surveys in those states which for nearly a year had shown Trump either tied with or slightly leading Democratic President Joe Biden.

Biden dropped out of the White House race last month and endorsed Harris instead.

The polling also showed that voters still prefer Trump on the issues of the economy and immigration, though Harris had a 24-point advantage when voters were asked whom they trust on the question of abortion.

The Trump campaign pushed back against the new polls, questioning their methodology and suggesting they were released "with the clear intent and purpose of depressing support for President Trump."

Much can change in the nearly three months before the November 5 election.

Democrats, in any case, have taken heart in the surge of enthusiasm that has greeted Harris' candidacy, with many expressing relief after 81-year-old Biden stepped aside.

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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' 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.