Chuka Umunna feels more at home as a Lib Dem MP than he ever did as a Labour politician, party leadership candidate Sir Ed Davey has said.
The Kingston and Surbiton MP said local branches of the Labour Party being overtaken by the hard-left meant some London MPs were having a horrible time.
It comes as more allegations of anti-Semitism emerged following the broadcast of a BBC Panorama documentary.
The programme accused senior Labour members of interfering with local investigations. Labour called the investigation a smear against the party.
Steatham MP Chuka Umunna left Labour earlier this year, claiming a "culture of fear" exists within the party.
He has since joined the Lib Dems, after briefly helping found Change UK, and is helping the party to campaign against leaving the European Union.
Sir Ed told The Late Debate: "Chuka has felt more at home in our party than he ever did in the Labour Party.
"The way that some of the constituency Labour parties have been overtaken by the hard-left. They are giving good Labour MPs a very hard and horrible time.
"Chuka's said he's never felt so welcome, and he absolutely is." The Lib Dems made huge gains from Labour and the Conservatives in the European elections earlier this year.
The party has been the only major political party to openly oppose leaving the EU and has been campaigning for a second referendum.
Sir Ed is running to replace Sir Vince Cable as the leader of the Lib Dems, saying he's the candidate that can help unite the country following the Brexit vote.
He added: "I'm very worried about the divisions in our country, the rise in hate crime here in London, but also the divisions with left behind communities and regions, where we've seen a lot of inequality.
"We have to tackle that. If you're a true remainer you've got to reach out to the Leave community and show that we've listened and we will heal our country."
Jo Swinson, the MP for East Dunbartonshire in Scotland, is also running for the leadership.
The contest has been overshadowed by Boris Johnson and Jeremy Hunt's battle to take control of the Conservative Party and therefore become the next Prime Minister.
Sir Ed said: ""Jo and I are friends. Isn't it interesting to see a leadership fight where people are decent to each other?
"If we were as rude as Jeremy Hunt or Boris Johnson there'd be more coverage. I think it's a good thing that people can agree, but put forward a different perspective like how you stop Brexit.
"Clearly the Liberal Democrats are the Remain party in British politics."