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She’s not even standing in this election.

But Nicola Sturgeon could become the second most powerful person in the United Kingdom.

She’s the leader of the Scottish National Party (known as the SNP) – which campaigns for Scotland to leave the UK and become an independent country.

They seemed to suffer a setback last year, when Scottish people voted against independence in a referendum.

But the referendum actually led to a massive boost in support for the SNP.

And they could become partners in the UK government after the general election on May 7 – a bit like the Liberal Democrats did after the last one.

The latest opinion poll found that 49 per cent of Scottish voters say they’re going to vote for the SNP.

Labour are in second place – but only 25 per cent Scottish voters say they plan to vote for them.

It means a lot of Scottish seats currently held by Labour are expected to go to the SNP.

In fact, some experts think the SNP could win as many as 55 of the 59 seats in Scotland.

How opinion polls say Scotland will vote YouGov/Times poll

But of course, only Scottish voters get a chance to vote for – or against – the SNP. They don’t stand candidates anywhere else.

And their leader, Mrs Sturgeon, isn’t even standing in Scotland in this election.

She’s already a member of the Scottish Parliament in Holyrood, Edinburgh (and Scotland’s First Minister), and doesn’t want to come down to London to sit in the House of Commons.

So what would the SNP do if they have enough MPs to hold the balance of power after May 7?

1) The SNP say they will only work with Labour

So even if the Conservatives come first and Labour come second, they will help Labour leader Ed Miliband become Prime Minister, if they have enough MPs.

This is very unlikely to involve a full-blown Coalition, like the Conservatives signed with the Liberal Democrats. But there would have to be some sort of agreement between the SNP and Labour.

2) The SNP want the UK Government to stop cutting spending on services

Instead of making cuts, they want the Government to spend a little more instead.

3) the SNP are against renewing Trident, the UK’s nuclear weapons system

A decision on whether to spend up to £20 billion on new nuclear weapons is due next year.

4) the SNP want to stop any further private sector involvement in NHS services

In her own words: SNP Leader Nicola Sturgeon explains what her party stands for

Conservatives say this would all be bad for the UK – and is a reason for voting against Labour, to keep the SNP out.

But it’s always possible that some English voters will actually like the policies that the SNP is offering.

poll loading What do you think? Would the SNP be good for England? 0+ VOTES SO FAR YES NO

Read our guide: What you need to know about the general election

The battle for power

Our interactive poll-of-polls draws on every opinion poll published by each of the main polling companies over the last 100 days: