A major new poll puts Kamala Harris ahead of Donald Trump in three key swing states, signaling a dramatic reversal in momentum for the Democratic party with three months to go until the election.

The vice-president leads the ex-president by four percentage points in Wisconsin, Pennsylvania and Michigan, 50% to 46%, among almost 2,000 likely voters across the three states, according to new surveys by the New York Times and Siena College.

The polls were conducted between 5 and 9 August, in the week Harris named midwesterner Tim Walz, the governor of Minnesota and a former high-school teacher, as her running mate on November’s Democratic ticket.

It provides the clearest indication from crucial battleground states since Joe Biden pulled out of the race and endorsed Harris amid mounting concerns about the 81-year-old’s cognitive wellbeing and fitness to govern for a second term. The results come after months of polling that showed Biden either tied with or slightly behind Trump.

Harris is viewed as more intelligent, more honest and more temperamentally fit to run the country than Trump, according to the registered voters polled.

The findings, published on Saturday by the New York Times, will boost the Democrats, as Harris and Walz continue crisscrossing the country on their first week on the campaign trail together, holding a slew of events in swing states that are likely to decide the outcome of the election.

On Saturday, the candidates held a rally in Las Vegas, Nevada, a state the Biden-Harris ticket won by more than two points in 2020.

While only a snapshot, Democrats will probably be heartened to see that 60% of the surveyed independent voters, who always play a major role in deciding the outcome of the race, said they are satisfied with the choice of presidential candidates, compared with 45% in May.

The swing appears to be largely driven by evolving voter perceptions of Harris, who has been praised for her positivity and future-focused stump speeches on the campaign trail. In Pennsylvania, where Biden beat Trump by just more than 80,000 votes four years ago, her favorability rating has surged by 10 points since last month among registered voters, according to Times/Siena polling.

Harris will need to win Wisconsin, Pennsylvania and Michigan – crucial battleground states that Biden clinched in 2020 – if the Democrats are to regain the White House.

The latest polls will probably further anger Trump, whose few recent campaign events have largely been dominated by ire – and apparent disbelief – at the rapid shift in momentum since naming JD Vance, the Ohio senator and former venture capitalist, as his running mate amid a celebratory atmosphere at the Republican national convention less than a month ago.

Vance, who has been derided as “weird” by the Democrats as he doubles down on 2021 comments about the US being run by “childless cat ladies”, is broadly viewed unfavorably or unenthusiastically by the majority of independents, Democrats and registered Republicans, the new poll found.

But Democrats still have work to do to communicate Harris’s vision for the country. The poll found that 60% of registered voters think Trump has a clear vision of the country, compared with only 53% when asked about Harris.

Crucially, Trump is also still leading when it comes to confidence over handling the economy and immigration – two of the three key issues for voters, according to polls.

Still, Harris has a 24-point advantage over Trump when it comes to abortion, an issue which Democrats hope will help get out the vote in key swing states such as Arizona and Wisconsin. Harris is also viewed significantly more favorably when it comes to democracy than Trump, who continues to face charges related to his alleged role in subverting the 2020 election results and the 6 January insurrection in Washington.

In a statement to the Times, Tony Fabrizio, the Trump campaign’s chief pollster, said the new polls “dramatically understated President Trump’s support”, citing surveys conducted in the days before the 2020 election that overestimated the margin of Biden’s victory.