Josh Shapiro, who was nominated this week as the Democrats’ candidate for governor in the electorally critical state of Pennsylvania, has accused his Republican rival of intending to override the democratic will of voters and pick his own winners in future elections.

Shapiro launched his attack on Doug Mastriano in an interview on CNN’s State of the Union on Sunday. He called Mastriano, a far-right state senator, “dangerous and divisive” and warned that were he to become Pennsylvania’s governor he could wield power to choose his own slate of presidential electors as a means of overturning the results of the 2024 presidential election.

“Senator Mastriano has made it clear that he will appoint electors based on his belief system,” Shapiro said. “He is essentially saying, ‘Sure you can go vote, but I will pick the winner’. That’ is incredibly dangerous.”

Fears about the anti-democratic leanings of Mastriano have rippled across Pennsylvania and through the country since he won the Republican primary last week. Were he to go on to defeat Shapiro, the state’s current attorney general, in November he would have considerable powers at his disposal to support what would in effect be an insurrection.

As governor, he would theoretically be able to refuse to certify the results of an election even though it had been conducted freely and fairly. He would also have the power to appoint Pennsylvania’s secretary of state – the position that controls all elections in the state.

Donald Trump endorsed Mastriano for the governor nomination shortly before the primary. The move was seen as rewarding the candidate’s loyalty in backing the former president’s attempt to cling to power illegitimately in 2020 – as well as paving the ground for a possible similar attempt at insurrection in 2024.

Mastriano was one of the most avid proponents of Trump’s “big lie” that electoral fraudsters stole the 2020 race against Joe Biden from him. He was present at the US Capitol on 6 January when Trump supporters and white supremacist extremists made their violent attempt to throw out the election results and keep Trump in office.

“Senator Mastriano wants to take us to a divisive and dark place,” Shapiro told CNN. “He has openly talked about, if he were governor, with a stroke of a pen doing away with voting machines which had votes that he didn’t agree with.”

Pennsylvania has been a vital swing state in recent presidential elections. Trump won the commonwealth by 44,000 votes in 2016, but he lost it to Biden four years later by 82,000 votes.

Mastriano is seen as being so extreme by Democratic strategists that the Shapiro campaign went to the lengths of running adverts during the primary that appeared to boost the Republican state senator – presumably on the principle that his far-right tendencies would make him easy to beat in November. The ad called Mastriano “one of Donald Trump’s strongest supporters” and said if he won the Republican nomination “it’s a win for what Donald Trump stands for”.

Shapiro was asked by CNN whether the move was an irresponsible attempt to help a candidate “because you think you can beat him”. The Democratic nominee denied the claim, saying he ran the ad as a way of getting an early start on the general election campaign.