Donald Trump has told his supporters at a rally in Lititz, Pennsylvania that he “shouldn’t have left” the White House after the 2020 election and, following the release of new opinion polls, claimed the firms behind them are corrupt.

The former president also spent large swaths of his rally in Pennsylvania suggesting, without evidence, that the election system, polling firms and media were corrupt and conspiring against his campaign – the latest signal that the former president is gearing up to contest the results of the election if he doesn’t prevail.

Trump, whose teleprompter had technical difficulties, devoted copious time to denounce early voting, even though his advisers have pointed to higher turnout among Republican early voters to suggest his campaign has momentum. He said he believed voting should be held only on election day, with results certified immediately that night.

“Everyone’s afraid to damn talk about it, and then they accuse you of being a conspiracy theorist,” Trump said.

Trump went on to say he believed the US voting system was worse than those in developing countries, and complained that his top campaign aides needed to spend time strategising about challenging election procedures rather than campaign spending or rally locations.

“We had the best border, the safest border,” Trump said of his time in the White House, according to The New York Times. He said the economy had been in good shape, before mentioning the chart he had been pointing to featuring immigration statistics when he was shot at during a rally in Butler, Pennsylvania, in July.

“It said we had the safest border in the history of our country the day that I left.

“I shouldn’t have left, I mean, honestly,” Trump continued, adding, “We did so well, we had such a great –” at which point he cut himself off. Then he immediately noted, “So now, every polling booth has hundreds of lawyers standing there”.

Before the rally, Trump had featured a slate of high-profile female supporters as he looked to cut into Democrats’ advantage. Speakers included former professional race car driver Danica Patrick, former ESPN anchor Sage Steele, former Democratic congresswoman Tulsi Gabbard and Arkansas Governor Sarah Huckabee Sanders, who served as Trump’s White House press secretary.

The Trump campaign has sought to exploit comments by billionaire Mark Cuban, a top Harris supporter, who said on chat show The View last week that Trump is “never” around any “strong and intelligent women”.

Trump, who has been found liable for sexual abuse, has often undercut his own efforts – including suggesting at a recent rally that women should be protected “whether the women like it or not”.