The resignation of Stephen Phillips as a Conservative MP has just increased the influence of Northern Ireland unionists at Westminster.
There are 11 unionist MPs from the Province, who will now play a key role in whether Theresa May survives as prime minister, and whether Brexit happens.
There are only 328 Tory MPs in the House of Commons out of 650. Zac Goldsmith quit as Conservative MP for Richmond over the decision to expand Heathrow. He will now fight as an independent and might well win, but the Tories are not standing against him so the Liberal Democrats could be win the protest vote (they won a lot of it in the by-election David Cameron’s old seat Witney).
If all the opposition party MPs voted against the government in a vote of no confidence or a key Brexit division, the Tories would only win by a majority of six.
So it would only take three Conservatives to switch sides to bring down Mrs May.
This could easily happen. MPs are much more independently minded than they once were, as the actions of Mr Goldsmith and Mr Phillips have shown. Mrs May now has enemies such as Nicky Morgan after her ruthless ministerial reshuffles on arriving in 10 Downing Street.
However, the arithmetic for the prime minister is notably less fraught if the two UUP MPs, eight DUP and Sylvia Hermon back her in key votes. If so more than a dozen Tories can rebel.
But even so, there might be that many Tories who are so opposed to Brexit that they put that view above party.
It is often said that Mrs May could go to the country now because she is likely to win an increased mandate given that Jeremy Corbyn is entrenched as a Labour leader.
I think there are problems with that assumption.
First, some Labour voters who have made their point over Brexit could return to the party (this is unlikely, because the sort of British nationalism that led them to back Brexit is rejected by Corbyn).
Second, the Liberal Democrats could revive strongly at the next general election, if some of their past voters return to the fold, feeling that they made their point in 2015.
Third, the Conservatives could find that they are up against anti-Brexit ‘unity’ candidates, as seems to be happening in the Richmond by-election.
So Mrs May is not likely to want an election. The unionist MPs will be crucial in ensuring that she does not face one.
The abstention of the four Sinn Fein MPs also helps the numbers for Mrs May
Martin McGuinness has not ruled out the possibility that they will take their seats. The temptation must be very strong, given how it would change the narrow balance in key votes on Brexit.
• Ben Lowry (@BenLowry2) is News Letter deputy editor