Police are investigating the "very serious" allegation, and Premier Steven Miles has slammed the attack on a man he calls a good mate, despite their political differences.
Gregory candidate Sean Dillon was campaigning in Winton in the state's west and was about 20km from home when he came upon a car that appeared to have broken down.
"I pulled over to look at to see whether I could help ... and I was physically accosted," he told Brisbane radio 4BC on Thursday.
The men in the car allegedly told Mr Dillon to "keep his union-bashing mouth shut", picked him up, threw him onto the hood of his vehicle and punched him.
He said the men also threatened his family, telling him to spend more time with them instead of on the road.
"(There was) some pretty explicit instructions about what they thought around my continued involvement in politics," he said.
Mr Dillon said the "horrific" physical attack appeared to be a political shakedown.
"It was the last thing that you expect as a political candidate or a politician of any colour is that sort of attack," he said.
"You brace yourself for a very strong discourse with people who are not comfortable within politics, but to experience the physicality of it and the threatening of your family is next level."
Queensland Police Commissioner Steve Gollschewski confirmed the incident was under investigation.
"Being a public official isn't an easy gig; we want good people to put their hand up to be in public office at all levels and they should be treated with respect," he told reporters.
"That's not what Queensland is about.
"That kind of alleged threatening behaviour or any kind of criminal behaviour where another individual is targeted, isn't acceptable."
Police Commissioner Steve Gollschewski says public officials need to be treated with respect. (Darren England/AAP PHOTOS)
Mr Miles called out the brazen attack.
"There's clearly no place for that kind of behaviour in our democracy," he said in Mackay.
Mr Miles also revealed he and Mr Dillon are good mates and he "wouldn't say that about many LNP candidates".
"For an LNP candidate, he's a good bloke," he said.
Mr Crisafulli called the attack "harrowing", casting blame on Labor for the incident.
"There's been plenty of lines crossed during this election campaign from the Labor Party but this one is a level of crossing the line that it's so far that you can't see it," he told reporters in Townsville.
"To have somebody roughed up because they put their hand up to serve and to think that could happen in modern Queensland is chilling and it says everything that is wrong about aggression in politics."
More than 450,000 Queenslanders have already made up their mind on the next leader of the state by casting ballots ahead of the polls.
The two leaders are still on the hustings to convince the remaining three million voters to tick one on their ballots for them.
Steven Miles and David Crisafulli are still battling to win more than three million votes. (Darren England/AAP PHOTOS)
Mr Crisafulli, who is leading the polls, has vowed to make false political advertising unlawful if elected on October 26.
"Heaven knows we need it based on some of the things I've seen coming out of the Labor Party this campaign," he said.
There have been a plethora of scare campaigns in recent weeks.
The LNP has been claiming a Labor government would introduce a "patients tax" - a slogan for the payroll tax on general practitioners that would be passed on to customers - which the government has promised to scrap.
Labor has been claiming the LNP will privatise satellite hospitals and remove progressive coal royalties.
When asked to commit to a truthful campaign for the next nine days during the second leadership debate, both Mr Crisafulli and Mr Miles claimed it was "not a scare campaign if it is true".
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