A row about anti-Semitism is being used by Jeremy Corbyn's opponents within Labour to undermine his leadership, Unite chief Len McCluskey says.
Mr McCluskey, whose union is Labour's biggest donor, attacked a "cynical attempt to manipulate anti-Semitism for political aims".
Another Corbyn ally, Diane Abbott, said party members had been "smeared".
It comes after controversial comments by ex-mayor Ken Livingstone and MP Naz Shah saw both suspended from the party.
Mr Corbyn told a May Day rally in London his party stood "absolutely against anti-Semitism in any form".
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Labour is holding an independent inquiry into anti-Semitism and other forms of racism in its ranks - but the leadership's response to the allegations has been criticised by some Labour MPs.
Ex-minister Ben Bradshaw told the Sunday Politics the issue had been allowed to "drag on".
Labour donor Michael Foster, who gave the party £400,000 before the general election where he also stood as a candidate, said some members of the party's leadership "vilify Jews as part of their political shtick".
'Mood music'
Mr Livingstone, the former London mayor, was suspended from the party after saying Adolf Hitler had supported Zionism in the 1930s "before he went mad and ended up killing six million Jews".
Mr McCluskey dismissed the controversy as "mood music" being exploited by political enemies of Mr Corbyn.
Image copyright Getty ImagesSpeaking to John Pienaar on BBC Radio 5 Live, he said Labour had been right to suspend Mr Livingstone for his "bizarre" comments.
But the Unite chief said the row had been "got up" by the right-wing press, "aided and abetted by those Labour MPs who seem to the first thing they do when they wake up of a morning is think of what stick can we beat Jeremy Corbyn with".
Mr McCluskey said he did not think there would be a formal leadership challenge to Mr Corbyn, but added that any MP "stupid enough to try" would have to "be accountable for their actions".
Speaking on the BBC's Andrew Marr Show, Ms Abbott, the shadow international development secretary, said Mr Livingstone's remarks had been "extremely offensive", but defended the party's handling of the row.
She said there had been 12 reported cases of alleged anti-Semitism within the party since Mr Corbyn became leader and each had led to a suspension - including Mr Livingstone who was suspended "within hours".
"It's something of a smear against ordinary party members to say the party has a problem with anti-Semitism," she said.
But Labour's candidate for Mayor of London, Sadiq Khan, warned the row engulfing his party had harmed his chances of being elected.
Mr Khan told the Observer Mr Livingstone's comments had made it "more difficult" for Jewish voters to support him in next week's election.
Image copyright PAAnd Labour MP for Liverpool Riverside Louise Ellman, said Mr Corbyn's handling of the row had been "much too slow".
Ms Ellman, who is Jewish, said she had been subject to anti-Semitic abuse herself at a meeting of the local party. A complaint is being investigated.
Senior figures within Israeli politics also expressed their concerns about the party.
The new Israeli ambassador to London said elements of left-wing parties were "deluding themselves" if they presumed there was no problem with anti-Semitism.
Mark Regev said comments in the last few weeks had crossed from criticism of Israeli government policies to "demonising and a vilification of" the Jewish state and perpetuating racist stereotypes.
Israel's Labour leader Isaac Herzog also said he had been "appalled and outraged by the recent examples of anti-Semitism by senior Labour Party officials".
Mr Livingstone's comments were "horrific and unthinkable for a British politician in the 21st century", he added.
'Telling the truth'
The anti-Semitism row erupted last week when Bradford West MP Naz Shah was suspended over comments she had made about Israel.
Mr Livingstone defended her in a BBC interview, saying that she was not anti-Semitic.
He went on to say: "When Hitler won his election in 1932 his policy then was that Jews should be moved to Israel."
Mr Livingstone was subsequently suspended but stood by his comments, saying he was "not sorry for telling the truth".