Joe Biden used a Labor Day speech in the battleground state of Wisconsin to endorse the expansion of unions, reiterating his election promises to be the “most pro-union president” in American history.

The US president argued in Milwaukee that a skilled, unionised workforce would help the US regain its place as a world leader in infrastructure and manufacturing.

Drawing on Franklin D Roosevelt’s explicit support for unions during the New Deal, Biden said: “I am encouraging unions … we need key worker protections to build an economy from the bottom up and middle out. I am sick and tired of trickle-down economics.”

Biden’s comments come amid a major resurgence for the labor movement in the US, with more support for unions than at any time in the past 60 years, especially as low-paid workers across a range of industries try unionising.

Earlier on Monday, Biden came out in support of a proposed law in California, the Agricultural Labor Relations Voting Choice Act – currently on Governor Gavin Newsom’s desk – that would make it easier for farmhands to organise.

“The least we owe them is an easier path to make a free and fair choice to organize a union,” Biden said.

The Labor Day holiday in an election year typically marks the start of the final sprint before the November vote. With so much at stake in this year’s midterm elections, Biden and Republican leaders are revving up the rhetoric.

There is also fevered speculation about whether Donald Trump will announce, before the election, a fresh run for the Republican nomination to recapture the White House in 2024, while he is embroiled in a host of criminal and civil investigations, from New York to Georgia.

In Wisconsin, Biden again attempted to distinguish between the type of mainstream Republicans whom he has previously worked with and the “extreme right, Maga Republicans, Trumpies”, he said, who “pose a threat to democracy and economic security, and embrace political violence”.

His use of the word “Trumpies” lit up social media. Biden in office has largely avoided referring to his predecessor by name in public or taking direct aim at his loyalist voter base.

But last month he referred to the phenomenon of extremist Republicans hewing unshakably to Trump’s “Make America great again” nationalist agenda amid encouragement of “political violence” as “semi-fascism”, then last week said the US was in a battle for the soul of the nation.

On Monday he said: “You can’t be pro-insurrection and pro-democracy,” referring to defenders of the January 6 attack on the US Capitol by extremist Trump supporters hoping to overturn Biden’s victory.

Biden continued on the campaign trail from Milwaukee to Pittsburgh for his third visit to Pennsylvania in a week – underscoring the importance of the swing state, which the president, a Pennsylvania native, won back for the Democrats in 2020. Trump, who won Pennsylvania in 2016, rallied there on Saturday.

After months of dire polling, the signs are more positive for Biden and the Democrats after a spate of legislative and policy wins, including getting a historic bill to tackle the climate crisis and healthcare costs over the line.

The US supreme court’s decision in June to overturn the right to abortion also seems to be galvanising the Democrat base, independent and swing voters, especially women, which could hurt Republicans at the polls.

In Wisconsin, Biden listed some of his administration’s key victories for workers and ordinary Americans through last year’s American Rescue Act (Arpa) and most recently the Inflation Reduction act (IRA) – without any Republican support.