The treasury spokesman became the only candidate to argue the party should have adopted an out-and-out Remain position – because its “triangulating” damaged the Labour leader’s standing.
Mr Lewis – an early supporter of a Final Say referendum – described Brexit as “a hard-right political project” that Labour “had to fight with all its vigour”.
“If you pick a side, you have to then argue for that side, and argue for it comprehensively,” he told BBC Radio 4’s Today programme.
“One of the key USPs of Jeremy Corbyn was that he was an authentic politician. I genuinely believe that he was, but on the biggest political crisis of British modern politics we were unable, via Jeremy, to be able to take a position.
“I think actually we should have stood our ground and argued our case.”
The stance is in marked contrast to that of the other leadership candidates, including Rebecca Long Bailey, who finally broke her silence to confirm she plans to run.
The shadow business secretary criticised the party’s Brexit position, saying “our compromise solution satisfied too few”, but she did not set out what the party should have done instead.
Mr Lewis said that his personal experience on the doorstep had shown him that Mr Corbyn’s leadership was a problem for many voters.
But he also pointed to problems in the party’s heartlands going back decades, saying: “For the last 40 years in this country there has been a decline in those votes in those areas where we collapsed in the general election.
“And this is something that has been going on for 40 years, and that includes over the period of 1997 to 2010. So to understand what actually went wrong with this vote, you need to look back further than just 2015, or 2010.
“It is like trying to work out the causes of the Second World War by going back no further than 1938. You have to go back to 1914, and possibly beyond.
“We have to work out why were we not able to connect to people in these heartlands, in the north, in the east Midlands? Why couldn’t we get our message across to them?”
Labour’s ruling national executive committee will meet next Monday to decide the format of the election, with the intention of installing the new leader by the end of March.
In her newspaper article, Ms Long Bailey pledged to champion “progressive patriotism”, while avoiding criticism of Mr Corbyn or of his handling of the antisemitism issue.
But, in evidence that doubts are growing on the Left about her credentials, Mr Lavery, the party chairman, also announced that he is weighing up a leadership bid.