Chuka Umunna has launched a scathing attack on his former leader, Jeremy Corbyn, accusing him of “lauding authoritarian regimes”.
The Liberal Democrat MP left Labour in March over its position on Brexit and handling of anti-Semitism allegations.
The foreign affairs spokesman used his speech at party conference to condemn Mr Corbyn for allowing his supporters to “abuse” and “vilify” his critics.
And he said neither the Labour leader or Boris Johnson were “fit to rule”.
The Lib Dems’ annual conference in Bournemouth has seen the party agree a new stance on Brexit.
Members voted overwhelmingly in favour of a manifesto pledge to revoke Article 50 – the formal process of leaving the EU – if they came into power with a majority government.
The party’s leader, Jo Swinson, said she would still be campaigning for a further referendum, but if an election was called, voters wanted a “straightforward” proposal to “resolve the issue of Brexit”.
BBC political correspondent Jonathan Blake said there was a feeling from some delegates that it was a big risk to take, and an “extreme position” that could see them accused of being anti-democratic.
Lib Dem MP Norman Lamb said his party was “playing with fire”, telling BBC Radio 4’s Today programme that the polarisation between Leave and Remain was “incredibly dangerous”.
“If we take this to the very limit in a situation where one side is vanquished entirely, I think there’s a real danger that we break the social contract in our country,” he added.
After leaving Labour earlier this year, Mr Umunna became a founding member of the Independent Group for Change.
But he left that party in June after poor results at the European elections, and joined the Lib Dems.
In his first speech at his new party’s conference, he said Labour “likes to think of itself as a champion of liberal values”, but it wasn’t under Mr Corbyn’s leadership.
“You cannot be a champion of liberalism if you are currently subject to a formal investigation by the Equality and Human Rights Commission for institutional racism against Jewish people,” he said.
“You cannot be a champion of liberalism when your leader’s supporters think it is acceptable to abuse, vilify and deselect anyone who dares to question the leader.
“And you cannot claim to be liberal when the political editor of the BBC needs to take a bodyguard to your conference.”
Mr Umunna criticised the Labour leader’s foreign policy positions, saying he acted as “an apologist for a hard-right Russian government that thinks it’s OK to poison people on British soil”.
He also accused him of “lauding authoritarian regimes in Venezuela and Iran” and “failing to support the prescription of Hezbollah as a terrorist organisation”.
“These people aren’t liberal,” he added.
The Lib Dem MP also criticised the prime minister’s approach to Brexit, saying he is “seeking to force through a catastrophic no-deal”, and “flouting the rules at home and abroad”.
He said Boris Johnson and the Brexit Party leader Nigel Farage were “the peddlers of hate and division”.
Mr Umunna added: “What is clear is we will not see the leadership on the world stage required from the new occupant of No 10.”
Instead, he praised his leader, saying Ms Swinson could “build and breathe a progressive breath of fresh air” into UK politics.