In the days following the 2017 general election, where Labour denied the Conservatives a majority, defying expectations of practically every mainstream pundit, Jeremy Corbyn called for the party to remain in “permanent campaign mode” and “on a general election footing”. It seemed that Theresa May’s precariously cobbled-together government could fall at any time, and that as a party we needed to be ready to capitalise on that. With the Conservatives on the ropes over Brexit, it looks like that moment is fast approaching. But that is put at risk by internal divisions crowding out our message, such as the latest campaign demanding Tom Watson’s resignation.

The Corbyn project has been around for only three years, but it’s already gone through many iterations. The first phase was the 2015 leadership contest, where Corbyn won a landslide on a wave of insurgent optimism. The second phase – between the second leadership contest and the 2017 general election – showed Corbynism to be both resilient and electorally viable, as the party gained in seats and share of the vote. Throughout both of these phases, attempts were made by sections of the parliamentary Labour party, the media and the commentariat, to delegitimise Corbyn’s leadership. There was little intellectual curiosity devoted to the project, as many assumed it was only a “blip”, with the belief that natural order would be soon restored.

The derision that many of us faced led to a siege mentality among supporters of Corbyn’s leadership, myself included. Being ideologically aligned to Corbyn, and believing that his platform not only provides the best opportunity for electoral success in the current context, but also that a transfer of wealth to the many poses a genuine threat to the economic elite meant attacks – often in bad faith – were inevitable.

But not all criticism is in bad faith. While some of Corbyn’s critics are undoubtedly using the problem of antisemitism as a means to attack Corbyn, that doesn’t mean antisemitism doesn’t exist among the Labour membership. Like Tom Watson, we should all want to eradicate antisemitism from the party. The deputy leader expressing these concerns is not grounds to call for his resignation, as many have, using the hashtag #ResignWatson.

Before the last general election, Corbyn’s critics in the parliamentary Labour party and among activists were divided into two camps: those who wanted Corbynism to be electorally viable, but didn’t think that was possible, and those who didn’t want it to be electorally viable because of an ideological opposition to what Corbyn stands for. The latter group would rather see a Conservative government led by May than a Labour government led by Corbyn.

Some of these people will use any means necessary to destabilise the leadership to prevent Corbyn entering No 10, even a “scorched earth” approach of taking the reputation of the party down with them. If you don’t believe me, take what Peter Mandelson said last year, describing his attempts “every single day” to “bring forward the end of [Corbyn’s] tenure in office”. If recent news reports are correct, they are prepared to start a new party if it means preventing a Corbyn-led government.

This scenario planning by the wreckers is an inevitable consequence of Corbyn entering the third phase of his leadership, where he is now a prime minister in waiting.

But the failure to tolerate good faith concerns expressed by some members will only embolden those who want Corbynism to fail. Most Labour party members do not see themselves as factional, they want Labour to win and to get into government. Momentum may have 40,000 members, but the party has 800,000 if you include affiliated supporters. Corbyn won over the majority of every section of the membership because most of us believed he was the best candidate and therefore had the best chance of winning an election.

Criticism will often be in bad faith from those who want to prevent a Corbyn-led government, but we have to be able to tolerate criticism in good faith, including from those won over by Corbyn after his performance in the 2017 election, such as Watson. We have to be able to distinguish between good faith criticism and attacks in bad faith.

We can’t retreat into the bunker – the siege mentality phase has passed. Now is the time to speak to the electorate, to deliver our positive and hugely popular message to the country. Accepting good faith criticism won’t harm us in the eyes of the voters, but unnecessary division will.

• Matt Zarb-Cousin is the former spokesman for Jeremy Corbyn