Most new PMs have fairly short "honeymoon" periods before things start going pear shapes, but I can't remember the last time when a new PM had members of his own party resigning from office because they couldn't work with him.
This is my own prediction for PM's prime ministership. It could end up being one of the shortest (if not the shortest one) in British history.
Johnson has got where he is by promising lots of things that he can't fulfill, like tax cuts for high earners combined with promises of spending. Now he will have to face reality.
He says he wants to renegotiate the Brexit deal with the EU. The EU have persistently said they will not renegotiate it. He wants to get rid of the Irish "backstop"; the EU have said that that's not going.
So he says he will get the UK out of the EU on or by 31 October, with or without a deal. But there will no deal except for the one negotiated by Theresa May, and Parliament have voted to prevent the UK leaving without a deal (which almost all economic experts, plus both the CBI and TUC, in an unlikely alliance, agree will be economically catastrophic for Britain).
So what are Johnson's options?
* Ask the Queen to pro-rogue Parliament so he can rule as a dictator. Whether the Queen says yes or no to this, it means she is being dragged into politics, which she has avoided all her reign. It will provoke the biggest constitutional crisis in 300 years.
* Threaten to withhold our £39 billion exit bill. This will amount to economic warfare with the EU, and would have unpredictable consequences. It would cause huge reputational damage to the country, affecting our ability to negotiate all these trade deals we're supposedly going to have around the world. The EU wouldn't take it lying down, they would retaliate in ways as yet unknown.
* Betray his promise and accept the Brexit deal, or climb down and ask for another extension to Brexit. That would be vintage Johnson but it would destroy his own reputation.
* Call a snap election, which he said he won't before Brexit, but he may be left with no choice. In a four way fight in England between the Tories, Labour, the Brexit Party and the Lib Dems, the Tories could face being hammered. Many Conservatives might refuse to stand on a platform of a no deal Brexit. The party could be split and might not survive.
All in all, Johnson may not even stay in power for the 99 days till Brexit.