Joe Biden will warn on Thursday that an “extremist” faction of the Republican party loyal to former president Donald Trump presents a direct threat to American democracy.

In a primetime address from Philadelphia, the city where American democracy was born, Biden is expected to cast the November midterm elections as a high-stakes battle for the “soul of the nation”, reprising a theme that animated his campaign for the White House in 2020.

The speech is part of a newly aggressive line of attack Biden has unleashed on Republicans ahead of the midterm elections, as his party enjoys a brightening political outlook helped by a string of significant legislative wins and building public backlash to the supreme court’s decision to end the constitutional right to abortion.

At a recent political rally in a well-heeled Washington suburb, Biden accused the Republican party of embracing “semi-fascism”.

Previewing Biden’s remarks, the press secretary, Karine Jean-Pierre, told reporters on Wednesday that the president believes “Maga Republicans” present “an extreme threat to our democracy, to our freedom, to our rights”.

Thursday’s primetime speech is the second of three visits by the president in less than a week to battleground Pennsylvania, home to several consequential races this election season.

In the US Senate race, Mehmet Oz, the Trump-backed heart surgeon turned celebrity doctor, is squaring off against the state’s lieutenant governor, Democrat John Fetterman, in a contest that could determine which party controls the chamber, evenly divided at present.

Meanwhile, Democrats have warned about the risks of Doug Mastriano, the far-right Republican nominee for governor, a leading figure in Trump’s efforts to overturn the results of the 2020 election in Pennsylvania who helped shuttle people to Trump’s rally in Washington on 6 January that preceded the attack on the US Capitol.

In Pennsylvania, the governor appoints the secretary of state, giving the next governor enormous sway over how the 2024 presidential election is conducted in the state. Mastriano faces Democrat Josh Shapiro, Pennsylvania’s attorney general.

In a speech not far from Biden’s birthplace of Scranton, Pennsylvania, on Tuesday, the president lashed “Maga Republicans in Congress” over their attacks on the FBI after agents seized boxes of classified documents from Trump’s Mar-a-Lago estate last month. The remarks were designed to counter Republican attacks on Democrats as “soft on crime”, with Biden casting his opponents’ rhetoric as a threat to law enforcement and the rule of law.

“The idea you turn on a television and see senior senators and congressmen saying, ‘If such and such happens there’ll be blood on the street’?” he said in Wilkes-Barre. “Where the hell are we?”