Scare tactics used by both sides of the referendum campaign have "brought out the worst in Westminster politics", Labour's John McDonnell will say.
The shadow chancellor will use a speech at the TUC in London to criticise Conservative MPs on all sides for resorting to "project fear".
Mr McDonnell will urge remain campaigners to instead focus on presenting a positive case for staying in the EU to win round voters.
He is expected to say: "I want to rescue the debate from the negativity and project fear coming from all sides of the Tory party.
"The referendum resulted from the splits in the Conservative Party and their fear of Ukip, and as a result the debate has degenerated and brought out the worst in Westminster politics.
"People want facts, but more importantly they want to see the progressive alternative - not just negative Tory infighting.
"It's time to turn this debate around, drive out the politics of despair and offer a vision for Britain and Europe, one where we protect workers' rights, tackle tax avoidance, get to grips with climate change and protect our industries like steel.
"This is a vision of Europe based on hope and solidarity."
Both sides of the campaign have repeatedly criticised each other for using scare tactics to win voters in the run-up to next month's referendum.
Lead Leave campaigner Boris Johnson was accused of making "hysterical claims" by Labour's Yvette Cooper when he compared European Union efforts to build a superstate to Hitler's attempt to dominate the continent.
Prime Minister David Cameron was mocked when he used a speech to warn peace and stability would be put at risk by Britain quitting the EU.
