Our poll suggests the Tories would have a small majority after an election – but fall far short of one if pro-EU voters act together

Tony Blair’s victory in the 1997 general election was widely expected. But the size of Labour’s majority was not. Most predictions ranged from 80-130. The final tally was 179.

What made the difference? Not the size of Labour’s lead in the popular vote. This was actually less than most polls projected. The biggest factor was tactical voting. In seat after seat, thousands of voters supported not their first-choice party, but the candidate best-placed to defeat the Conservatives. Tactical voting cost the Tories about 30 seats: 18 won by Labour, 12 by the Liberal Democrats.

In the coming election, tactical voting could matter even more: it could determine whether the majority in the new parliament is for or against Boris Johnson’s plans for Brexit.

Sunday’s Observer/Opinium poll makes the point with savage clarity. On normal assumptions of a uniform swing across England and Wales (and allowing for the very different voting patterns in Scotland), this is what the new House of Commons would look like:

Conservative 325 (up 7 since 2017)

Labour 221 (down 41)

Liberal Democrat 31 (up 19)

SNP 50 (up 15)

DUP 10 (no change)

Others 6 (no change)

The Conservatives would have an overall majority of seven (assuming that Sinn Féin’s MPs, excluded from the table, continue to boycott Westminster). If the DUP continues to support Johnson’s Brexit policy, the government would have a majority of 27 on the big issue of our time.

However, if tactical voting had the same impact as in 1997, the Tories would have only 295 MPs. Even with the support of the DUP, they would fall well short of the number needed to remain in office. The combined total of the anti-hard-Brexit parties would be 338. If they were able to join forces, they would have a majority over the Tories and DUP of 33.

Those figures are inevitably approximate. The shifts in votes will vary, sometimes wildly, from seat to seat. The calculations here assume that the seats with above-average swings will broadly balance out those with below-average swings.

St Albans is a case in point. On current national trends, it should be a Liberal Democrat gain, even without tactical voting. But if it happens to be one of the seats with a below-average underlying swing, the Conservatives might see their 6,109 majority cut but not wiped out, unless enough Labour supporters vote for the Lib Dems’ Daisy Cooper.

Could tactical voting have an even bigger impact than in 1997? It’s possible: although there was some media coverage of its potential – not least in the Observer – there were no organised campaigns, and no social media, to get the seat-by-seat message out to voters. Much of the impact was down to local, informal agreements between Labour and the Lib Dems. For example, in East Sussex, the Lib Dems made little effort in Labour’s target seats of Hove and Brighton Kemptown, while Labour activists were all but invisible in the Lib Dem target of Lewes. The Conservatives lost all three seats; without tactical voting they might have lost none of them. This time, with a combination of similar informal deals and effective tactical voting campaigns, it is possible that the Conservatives could lose 40, or even 50, seats they might otherwise hold.

Moreover, for those wanting to stop a hard Brexit, turbocharged tactical voting might be necessary. The Brexit party still has 13% support. If the Tories can squeeze this down – and/or agree an electoral pact with Nigel Farage so that the Tories have a clear run in their key seats – this would plainly boost the Conservative seat tally.

Based on the Tories’ current 10-point lead, a tactical voting loss of just 15 seats would ensure that the Conservatives and DUP together lack the numbers in parliament to pursue Johnson’s Brexit strategy. But if the Brexit party is no longer a threat in the Conservatives’ target seats, it is quite possible to see the Tories winning 350 or 360 seats, in the absence of tactical voting.

Although tactical voting looks like a numbers exercise, apparently remote from what Aneurin Bevan called “the poetry of politics”, its democratic significance is potentially huge. In today’s poll, the total support for the Conservatives, Brexit party and Ukip is 48%, while that for the parties ranged against them is 52%. Yet under our first-past-the-post voting system, the minority in the country could enjoy a majority in parliament, because one party dominates, and could have a near-monopoly of the pro-Brexit vote, while support for its opponents is divided.

The brutal truth about the coming election is that the scale of tactical voting could well determine whether or not the underlying public majority opposed to Johnson’s Brexit strategy is reflected in the new House of Commons.