Joe Biden has proposed criminal prosecution of companies that deprive workers of their rights to organise unions as part of what he called the corporate war on labour.

The former vice-president made a vigorous defence of the labour movement at a forum of Democratic presidential candidates in Iowa organised by the giant Teamsters union in partnership with the Guardian and the Storm Lake Times on Saturday.

All six candidates at the forum emphasised the close link between stagnant wages, surging corporate profits and four decades of legislation weakening labour unions, such as so-called right to work laws curbing union activity and collective bargaining. Biden said productivity in the past 40 years was up 64%, and with it shareholder dividends, while wages have risen just 8%.

“You’re the only ones who can keep the barbarians at the gate,” he told Teamster union members. “They only understand power. Power. Countervailing power. And you guys have been getting killed, organised labour … There’s been a war on labour’s house for the last 30 years.”

Two other frontrunners, Pete Buttigieg and Bernie Sanders, echoed the sentiment by saying they would pass federal laws guaranteeing the rights of union members to organise and negotiate. But Biden went further and said that as president he would consider criminal prosecution of the most egregious cases of companies denying workers their legal rights to organise.

Employers spend a billion dollars a year trying to bust unions from trying to start up in the first place Joe Biden

“Employers spend a billion dollars a year trying to bust unions from trying to start up in the first place,” he said.

Biden said relatively small fines imposed for such activities were no deterrent and that he would change the law to fine individual managers who order illegal anti-union moves. In some cases, he said, he would also look to pursue criminal prosecutions to protect workers rights.

“We should think about criminalisation,” he said. “If you’re engaged in trying to prevent people from voting in a polling place in a regular election, what happens? You can be held criminally liable. What is the fundamental difference between saying that a powerful employer can intimidate workers to not get engaged and vote for a union, that you have a right to vote for?”

Unspoken but hanging over the forum was the knowledge that many Teamster members voted for Donald Trump in 2016, frequently out of a mix of dislike of Hillary Clinton and a desperation to shake up an economic system not working for them. Sanders said they had been betrayed by Trump, who promised he was “going to stand with the working class of this country” and then served the rich.

Buttigieg took on those who still back Trump because they are told the economy is doing well while they are struggling to survive.

“When that sun comes up on the United States that first day Trump is no longer president, we’re going to have some big problems on our hands and one of those problems is going to be this economy, where the numbers on the page might look good, the stock market might look good, but most Americans aren’t feeling it,” he said.

“We are actually living shorter lives … For the gross domestic product to go up and American life expectancy to go down, that shouldn’t even be possible. That tells you what’s broken in our country right now.”

Buttigieg backed the common call for a $15 an hour minimum wage but said pay across the board is too low.

“The central problem in our economy is so abundantly simple that it is almost blindingly obvious, and that is this. People don’t get paid enough in the United States of America,” he said. “The productivity of our economy grew enormously over my lifetime. Shockingly little of that has gone to the workers who are actually generating the value.

“Yeah, there’s technology and globalisation. But let’s be very clear, this is the result of policy decisions. This is the result of political decisions beginning in the Reagan era that have disempowered workers and their ability to command more of the fruits of their labour.”

Pete Buttigieg speaks at the forum. Photograph: Andy Abeyta/AP

The clearest division between the candidates was on healthcare, an issue that divides union members too. The Teamsters has negotiated cheap, comprehensive health insurance with some employers, particularly big ones such as the car makers. But other members of the union pay much larger premiums for more limited coverage.

As Sanders pressed his plan for a single public health insurance scheme, Medicare for All, he met with a mixed reception. Some Teamster members cheered and clapped, others shook their heads. Buttigieg found a receptive audience among the sceptics for his proposal to introduce public health insurance for those who want it without forcing union members or others happy with private insurance to give it up.

The other proponent of Medicare for All, Elizabeth Warren, was not at the forum because her campaign said there was a scheduling conflict.

The Teamsters told the candidates that a survey of members showed large numbers are worried about the future of pensions underfunded by employers and at risk of collapse following a law permitting firms to scale back pension payments to retired workers. Sanders said that was a betrayal of a pact in which workers often agreed to forgo pay rises years ago in return for increased investment in their pensions. Biden said that as president he would order the Treasury to make low cost loans available to pension funds so they can pay out as promised.

Cory Booker, Amy Klobuchar and Tom Steyer also appeared. The candidates agreed that the pension crisis is further evidence of how corporate America corrupts politics to get the laws it wants at the expense of workers, and that has to change.