IT is not difficult to see why the SNP should in the end have been reluctant to pass up the possibility of an early election.

Standing at 40 per cent on average in the polls, the party can look forward to doing rather better than the 37% it won two years ago.

More importantly, both of its principal rivals, to whom it lost a significant number of seats last time, are in trouble.

At 20% in the polls the Conservatives look to be nine points down on where they were last time, while with just 17%, Labour are 10 points down.

Nearly all of the seats that eluded the SNP’s grasp in 2017 are marginal.

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As a result, if the current poll figures were to be realised in the ballot box, the party could be heading for something like 50 of the country’s 59 seats.

Even with a Liberal Democrat revival in prospect, that could be enough to ensure that the nationalists remain the third largest party in the House of Commons.

Even so, the SNP have taken something of a gamble in helping to precipitate an election.

The party wishes to stop Brexit and it hopes to hold an independence referendum in the second half of next year.

Neither of these is likely to happen if Boris Johnson were to succeed in winning an overall majority.

And while any gains that the SNP might make north of the Border would make it less likely that the Prime Minister will achieve that objective, they will count for little if the Conservatives makes significant gains from Labour south of the Border.

And that is, at present, what the polls suggest could happen.

The Conservatives now enjoy a 12- point lead in the polls, up from seven points just a few weeks ago.

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The difference is significant. With just a seven-point lead there was only a 50% chance that the Prime Minister would secure an overall majority in an early ballot.

However, if he were to secure a 12-point lead he would be almost guaranteed a majority – indeed he could well enjoy quite a healthy one of some 40 seats or so.

If the SNP gamble is to pay off, the Conservative lead over Labour needs to fall.

The principal foundation of that lead has been a squeeze on the Brexit Party vote; Leave voters have gradually been attracted by the apparent prospect that Mr Johnson might succeed where Mrs May failed.

However, despite negotiating a new deal and winning MPs support for it in principle, the Prime Minister has been unable to deliver his promise that Brexit would be delivered.

So one possibility is that some of those voters who in recent weeks have switched from the Brexit Party to the Conservatives decide to switch back again now that Brexit still has not been delivered.

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It has to be said though, that at the moment the polls suggest that relatively few Leave voters think that Mr Johnson is responsible for the failure to deliver Brexit, while the first polls to be taken since the progress of the Prime Minister’s Brexit bill was halted have not shown any clear evidence of a swing back to the Brexit Party.

The alternative is that Labour might increase its vote. That, after all, is what Jeremy Corbyn achieved in 2017. But arguably the political landscape is significantly different from then.

The principal reason why the party is low in the polls is that it has lost nearly one in five of those who voted for the party in 2017 to Jo Swinson’s much revived Liberal Democrats.

Almost all of the switchers are Remain supporters and Labour might find it difficult to win them back with a Brexit policy that most voters say they find confusing – though perhaps campaigns to persuade Remain voters to vote tactically for whichever candidate is best able to defeat the Conservatives locally may have some effect.

Meanwhile, in 2017, Mr Corbyn was arguably the only party leader with the ability to enthuse and persuade voters. This time he faces two of the most charismatic politicians in British politics, Mr Johnson and Nigel Farage, while Ms Swinson can be expected to put in a stronger performance than the hapless Tim Farron.

The Labour leader faces a more crowded field.

Still, maybe the Conservative lead will fall in the next six weeks. If it does, then the election might produce a hung parliament.

In those circumstances, there would seem little chance that the Conservatives would remain in office – the division over Brexit means that none of the smaller parties, including perhaps the DUP, will want to help Mr Johnson stay in power.

As part of the bargaining to form an alternative administration – most likely a minority Labour government – the SNP could hope to pave the way not only to a second EU referendum but also another independence ballot.

But for that to happen, the SNP are reliant on voters in England rejecting Mr Johnson’s blandishments and charm.

It is not a process over which the party has any control.

John Curtice is Professor of Politics, Strathclyde University and Senior Research Fellow, ScotCen Social Research and The UK in a Changing Europe.