The Conservative grandee Michael Heseltine has made an impassioned appeal to people to choose “the national interest” and ignore traditional party loyalties to help elect Liberal Democrats through tactical voting.

At a press conference alongside Chuka Umunna and Sam Gyimah, who sat as Lib Dem MPs in the last parliament having begun their careers with Labour and the Conservatives respectively, Lord Heseltine said he was still a Tory party member but planned to “lend my vote to the Lib Dems on this one issue” of stopping Brexit.

Heseltine, whose 25-year ministerial career included a stint as deputy prime minister under John Major, saluted Umunna, Gyimah and those who were planning to vote tactically.

“There are men and women whose commitment to our country, their sense of what matters to Britain, where the future of Britain lies, where the future of the younger generation lies, means that they have torn up their traditional loyalties,” he told the event in London.

Facebook Twitter Pinterest Chuka Umunna, left, with Michael Heseltine on Wednesday. Photograph: Chris J Ratcliffe/Getty Images

They had, Heseltine said, “put behind them the traditional loyalties to their party because they can see a greater loyalty to their country. And I will salute those people, the politicians who have risked everything, their careers, in order to be able to say: I stood by my convictions and I voted for my national interest.”

Heseltine’s appearance at a Liberal Democrat campaign event marked a new phase in the party’s push to defy weak poll numbers and gain extra seats on 12 December by seeking votes from remain-minded former Labour and Tory supporters.

In May, Heseltine had the Conservative whip in the Lords suspended after saying he had voted Lib Dem in the European elections. Resolving to stay in a party he joined in 1955, when Winston Churchill was prime minister, Heseltine said decades of loyalty had seen him tested over a series of issues.

“I was there for my party,” he said. “But throughout that entire time there was, for my party, one overriding argument that we deployed and argued for. It was the one theme of post-imperial Britain and it argued that we should target our destiny for this country at the heart of Europe.”

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He gave an emotional defence of UK membership of the EU as something that brought not only prosperity but had, over the decades, helped move the continent away from extremism.

“We are British in a united Europe, and we are proud about what that Europe has created. It’s not been just of forging rising living standards for the countless millions of our people, important though that has been,” he said.

“It has been that we have driven the words communism and fascism from the dictionary of European politics. We have seen the end of the colonels in Greece. We are proud to have been able to confront the extremes that so beset our European past. And we are proud to have shared our national power with nations like us so that we can collectively exercise an influence in the century that is unfolding, way above anything that we as nation states could do on our own.”

While Heseltine passed up the invitation in a subsequent Q&A to call Boris Johnson a liar – the prime minister was, he said, “the most flexible politician of this time” – he did call the Conservatives’ promise to get Brexit concluded within weeks one of the “more extreme and unreliable” claims of the election.

Even if Johnson passed his departure deal it would only “take us to where the real debate, the real punch-up, begins”, he said.

Heseltine dismissed the idea that Brexit would be made worthwhile by a swift trade deal with Donald Trump. “How many times is Mr Trump going to risk his fragile presidential majority in order to help his friend across the Atlantic?” he said.

“If you are going to gamble the future of the United Kingdom on the credibility of the generosity of Mr Tump and the far-sighted global wisdom of Nigel Farage, you will understand why I cannot take the journey with you. I have left your company – I will vote for British self-interest and those people who are standing in its name.”

Heseltine said he did not believe a vote for the Lib Dems could put Labour in power, saying there was “no way” Jeremy Corbyn would go into Downing Street. He was not advising people to vote Labour tactically, he said.