Privileging career prospects over the call of one’s conscience can terminally damage political credibility. That is the message those Conservative MPs working to prevent a no-deal Brexit will want their colleagues to hear when they go back to work in September.

Soon our elected representatives will have left the poolside and returned to the faded grandeur of Whitehall. Meanwhile those who stayed in Westminster this August, including the vast majority of ministerial special advisers, have been bracing themselves for the great Brexit battle 2.0. (Actually, it’s more like 10.0 – even giving Windows a run for its money. But never mind.)

It’s time to campaign until it is clear to this government that parliament won’t impose this risk on its people

So, will the first week of September usher in the mother of all parliamentary showdowns? That will depend in part on the decisions and priorities of those who have spent the summer attempting to reconcile their consciences with the prospect of no deal becoming a reality. Health secretary Matt Hancock managed it. So did cabinet ministers Amber Rudd and Nicky Morgan.

The consequences of no deal are now in the public domain, and they are grave. The leaked government dossier on Operation Yellowhammer suggests disruption to supplies of fresh food, fuel and medicine and the possibility of civil unrest. Just “bumps in the road” to Brexit says Michael Gove, the man in charge of no-deal planning.

I believe that Boris Johnson’s government is actively working to avoid a deal with the EU. I hope I am wrong. We will soon find out. What is undeniable is that this administration has done a brilliant job of branding and broadcasting its approach. The efficiency and potency with which the “do or die” message has been hammered home is bamboozling even the brave few who have thus far been vocal in their objection to leaving without a deal.

So what are the anti-no-dealers actually doing to prevent their nightmare coming true? There is much debate about how the legal requirement to leave on the 31 October can be upended. I’ll leave that to one side. That’s a practical question with a practical answer, and the various options will be played out soon enough. But this is also a question of principles and sticking to them. Or not.

Last week there was another leak. The former chancellor and my former boss, Philip Hammond, along with 19 other MPs, wrote to Johnson to remind him of his promise that no deal was an unlikely outcome – “a million to one” in the prime minister’s own words. The letter encouraged him to make good on these odds.

All well and good. But we have only two months before a hard Brexit becomes reality. So enough with the epistolary efforts. It’s time for senior Conservatives who know that no deal would be a disaster to get their hands dirty. It’s time to shout the message from the rooftops; to campaign and cajole until it is clear to this government that parliament won’t impose this risk on its people.

Last April, parliament successfully forced through an emergency bill to see off the threat of no deal under Theresa May.

But I am worried that, when the House of Commons returns in a fortnight, fewer and fewer MPs will muster the courage to speak in favour of blocking no deal and then to organise to prevent it. A majority in the Commons remains opposed to a hard Brexit. But I fear that, on the Tory side, too many may see blocking it as career suicide in the Johnson era.

How would we ever be able to trust these senior politicians again, when it would appear that a short summer break was all it took to convert them to the ruthless rhetoric of the prime minister’s approach?

Hammond has always said what he thinks. He’s authentically committed to the things he says he believes. As his communications director, my job was hair-raising at times. But it makes his job this September a straightforward one. What about former ministers Rory Stewart, Greg Clark, and David Gauke? Or Scottish Conservatives leader Ruth Davidson? They need now to step into the glare of the spotlight to lend the campaign against no deal some serious firepower.

Stewart, Clark and Gauke signed the Hammond letter. I expect they’ll continue to speak out against a hard exit. But they’ll not turn the worm.

Davidson may be the key to all this. Less than a week after Johnson was elected leader she made a public commitment that she wouldn’t support no deal. She says she also told Johnson this to his face in their first meeting following his win. Will she be as vociferous and vocal this autumn? I hope so.

But the most difficult questions are reserved for those who have chosen to remain inside the bunker. If you think leaving without a deal is a disaster, can you really have a clear conscience about staying in the cabinet?

Rudd has told colleagues she’s better able to influence Johnson’s hard Brexit instincts from inside government. But was the prospect of losing a second secretary of state job in such short order also a factor?

Rudd has sent mixed signals of late. First she declared that “I will play my part in … arguing strongly for respecting parliamentary sovereignty.” The next morning she doubled down on the no-deal message, writing in the Telegraph: “We will deliver Brexit by the end of October whatever the circumstances.”

Morgan’s website states she had concerns about “the uncertainty surrounding a no-deal Brexit,” and has made her views heard on the backbenches. But now, after three years of the UK stuck in a “holding pattern”, she says she “believe[s] we need to get the withdrawal phase concluded”. Careers, consciences and credibility: in the Conservative party this autumn all three will be tested as never before.

• Poppy Trowbridge was special adviser to Philip Hammond from September 2016 to July 2019