Senator Ron Johnson of Wisconsin was traveling to a campaign rally aboard Donald Trump's private jet in early April when he decided to broach the delicate issue of early voting.

As the Boeing 757 flew from Florida to Green Bay, Wisconsin, Johnson pressed the Republican candidate to use his speech to urge his supporters to cast their votes ahead of Election Day.

Early voting often draws a torrent of vitriol from Trump, who falsely claims it is vulnerable to fraud and cost him the 2020 election.

But Johnson is one of a number of senior Republicans – many of them in key swing states, like Wisconsin – who are worried that Trump's demonizing of early voting could torpedo his hopes of winning back the White House on Nov. 5.

"I encouraged the president to encourage Wisconsin Republicans to bank their votes," said Johnson, adding he didn't encounter any resistance from Trump. "I would recommend he get on board."

"We have to do everything we can to fully utilize the rules as they are written. Democrats certainly have," said the 69-year-old senator, himself a recent convert to early voting.

But not only did Trump fail to extol the importance of early voting at the April 2 rally, he told the crowd of more than 3,000 supporters that his goal was ultimately to limit voting to Election Day only, a message greeted with huge cheers.

And when Johnson – in his third Senate term - took to the convention center's stage to urge Republicans to vote early, he was met with tepid applause from a just few people.

Supporters of early voting say it increases turnout and avoids the problems of people being unable to vote on Election Day because of bad weather, logistical issues at voting stations or personal reasons. 

Voters rights groups say there is no data showing early voting can lead to fraudulent ballots being cast.