I think that a Labour or a Tory PM with a favourable view of PR (Blair, Hurd, and Alan Johnson all come time to mind) could give it away in the right circumstances as part of a coalition deal. In that situation, where the Prime Minister is leading the Yes campaign, rather than a discredited junior coalition partner, victory is achievable, provided the government is popular enough.
Was Blair ever really sympathetic to PR? He expressed a very skeptical view of it in a 1987
New Statesman article.
https://docs.google.com/document/d/1f7yY-0--dsEsv-M3cMrHFEv_92TkHYXEJ9VDFPfmNhw/edit
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"New Statesman 4 September 1987
"Tony Blair
"Electoral reform ain’t the answer
"UNTIL NOW, support for proportional representation was confined to the Liberal/SDP Alliance and to lone voices in the two main parties. But this year no fewer than 15 resolutions for the Labour Party’s annual conference favour proportional representation (PR) or some sort of electoral reform.
"It is argued that Labour has now lost three elections in a row, its support is irretrievably stuck at 30 per cent, give or take a few points, and that the only route back is to combine our vote with the other “anti-Thatcher” votes -- either through a pact with the Liberals (or whatever political Pushme-Pullyou results from the merger talks), or through PR.
"In other words, Labour’s new enthusiasts for PR put their case not primarily on grounds of constitutional principle, but as a strategy for power. The implications of their case are fundamental: that Labour cannot ever again win a majority of seats in parliament; and that what cannot be achieved through the front door of majority government can be bundled in by the back door of coalitions and electoral pacts.
"This view rests on dangerous delusions. It is obvious that we cannot confuse an “anti-Thatcher” majority with a “pro-Labour” one. What is less obvious, though as the Alliance unravels it becomes more so, is the inadequacy of the very notion of an “anti-Thatcher” majority. There are Alliance voters, probably even a majority, whose predominant characteristic is dislike of the Tories. But at present, there is a substantial minority of Alliance voters who are equally anti-Labour, as the Greenwich by-election showed. So, under PR, there is no guarantee that the 1987 election would have produced a Labour-led coalition. It was this fallacy of the cohesive “anti-Thatcher” majority that lay at the root of Tactical Voting 87’s difficulties in the general election.
"The real question for the Labour Party is why it is not achieving sufficient electoral support. It must face this question irrespective of whether we retain the present electoral system or change it, whether we stand for election alone or in a pact. The campaign for PR is just the latest excuse for avoiding decisive choices about the party’s future.
"A coalition still has to decide its economic policy, its industrial policy, what it intends to do about defence or foreign affairs or trade union law. An electoral pact must decide these things before it has even the prospect of power. Yet these are all the very decisions that Labour faces now. It can’t escape them by electoral reform or a pact, it can only postpone them. The reasons why people didn’t vote Labour at the last three elections won’t disappear through some mystical process of coalition.
"There is no decision that would be justifiable for Labour to make in order to win power in a coalition that it should not be making anyway for itself. Conversely, there is no decision that is unjustifiable for Labour to make alone that becomes justifiable by virtue of coalition. If a set of policies form an acceptable basis for coalition, they should be an acceptable basis for majority government..."
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True, New Labour in 1997 was theoretically committed to electoral reform--which did not necessarily mean true PR but could for example mean alternative vote ("instant runoff"). Yet Blair himself seems to have retained and indeed increased his earlier skepticism:
"Yet beyond this formal commitment to a referendum on electoral reform for Westminster elections, Blair's leadership, both in Opposition and subsequently in Office, was characterized by constant equivocation, with Blair himself simultaneously acknowledging the unfairness of aspects of the first-past-the-post system whilst remaining 'unpersuaded' of the case for a (more) proportional electoral system, not least because of the likelihood that 'small parties' would 'end up wielding disproportionate power' (Blair, 1996: 319-20).
"Indeed, in the wake of the 1987 election defeat, Blair had appeared even more dismissive of electoral reform, observing that 'Labour's new enthusiasts for PR put their case for not primarily on grounds of constitutional principle, but as a strategy for power', whereupon 'what cannot be achieved through the front door of majority government can be bundled in by the back door of coalitions and electoral impacts'. Yet this approach reflected 'dangerous delusions', according to Blair, for it sought to avoid the 'real question' as to 'why it [the Labour Party) is not achieving sufficient electoral support' (emphasis in original). As such, he alleged that 'The campaign for PR is just the latest excuse for avoiding decisive choices about the Party's future' (New Statesman, 4 September 1987). Certainly, Blair was concerned that some of his Labour colleagues were beginning to 'see PR as a panacea for their problems' (Sopel, 1995: 132).
"Reform of Britain's electoral system was, therefore, always likely to constitute one of the most equivocal parts of New Labour's professed commitment to constitutional reform, and so it proved. Tony Blair's equivocation enabled both proponents and opponents in the Labour Party to claim that he shared their view on the issue. Certainly, the scale of New Labour's election victories in 1997 and 2001 seemed to reinforce Tony Blair's reluctance to commit himself one may or another, although he certainly seemed to have become more sceptical about electoral reform during the course of his premiership..".
https://books.google.com/books?id=JsaHDAAAQBAJ&pg=PA34