Useful rule of thumb: May, without DUP, needs 1 Oppo MP vote for every Cons vote she loses. (Or an abstention for every abstention).
(With the DUP, has a margin. Could lose up to 10 more Cons MPs than she gains opponents).
[absent: Sinn Fein 7]
The most important issues in British political history have ended up being settled across parties. Here is a quick thread of 5 of the most consequential. But I may be forgetting others of similar importance.
First place, most important decision ever taken by a British govt. A cross-party vote in War Cabinet, not Commons.
May 1940: Clement Attlee & Arthur Greenwood back Churchill in a 3-2 vote against Halifax & Chamberlain to reject Mussolini's offer to mediate peace with Hitler.
(2) British entry to Europe/EEC
(White paper, Oct 1971 on principle of entry)
Heath has majority of 30, so15 rebels can wipe it out.
39 of his own MPs vote against
69 Labour MPs vote for the government (against a 3-line whip) and 20 abstain
Government majority 112
(3) Repeal of the Corn Laws, 1846
106 Conservative MPs vote with Prime Minister Robert Peel, 222 vote against him, on third reading.
Wins 327 to 229 (majority 98) with Whig votes
But loses an Irish coercion bill and resigns. His party out of office for a generation.
(4) The Parliament Act 1911
The two neck-and-neck 1910 elections wipe out Liberal landslide of 1906, so government needs Labour and Irish votes in the Commons and (finally) grudging Tory 'hedger' votes in House of Lords to finally remove the absolute veto of the upper house
(5), perhaps most controversially, the 1931 budget
Ramsay MacDonald's Cabinet is split 11-9 on the budget, and submits its resignation. He then forms a National Government with Conservatives and Liberals.
They go on to win 554 seats vs 52 for Labour in a snap election.
It seemed obvious to me, within an hour of the exit poll on 8th June 2017, that it would not be possible to deliver Brexit as a purely party issue. Yet that has remained the official plan for a majority-less government
Exit poll comment