“You can only drive a wedge so far between yourself and the people who put you in that position before your opponents start looking at their options,” the shadow Treasury minister told The Independent.
Mr Lewis warned Labour was “haemorrhaging” support from Remain voters and attacked senior party figures happy to finish second in next week’s European elections “as long as we beat the Tories”
“There are those who think Labour coming second is an acceptable position, as long as we beat the Tories, but that will open the door to them [the hard right] – an appalling situation.
“Hundreds of thousands, if not millions, of people will vote for these parties for the first time and, once they have broken that seal of always voting Labour, then doing so in subsequent elections becomes easier.”
He was among more than 100 MPs who signed a letter demanding Mr Corbyn endorse a public vote in the European elections manifesto, under the ‘Love Socialism, Hate Brexit’ banner.
“It would be a betrayal of our conference position and the vast bulk of our members and voters,” he said.
The Norwich South said it would be “very silly to speculate about leadership challenges”, given Mr Corbyn had won two contests, but insisted: “It would put in peril the political project so many of us want to see enacted.”
“I don’t want to see our party driven dragged back to the centre-ground of British politics, which has few answers to the radical challenges that face our society.”
In recent weeks, the Labour leadership has unveiled a blitz of domestic policies on housing, the wages of young people and green energy, among other key issues.
But Mr Lewis suggested the attempt to win a hearing until Mr Corbyn was willing to nail his colours to the mast on Brexit.
“Many Labour voters are among those most affected by austerity and have real fears about special needs education, the NHS, social care and children’s centres,” he said.
“But, on the Remain side, we can’t get to talk about these things because they feel so passionate about a confirmatory vote. It’s in effect become an identity issue.”