Recent polls show that the SNP has "become the national party of Scotland in all senses", Nicola Sturgeon said as she joined Alex Salmond on the campaign trail.

The SNP leader was with her former boss in Inverurie, in the Gordon constituency, where Mr Salmond is standing for a return to Westminster in a seat currently held by the Liberal Democrats.

In light of a much-praised performance in the second televised UK leaders debate and recent polls which indicate further SNP gains across Scotland, Ms Sturgeon said there is still "a lot of hard work to be done".

The First Minister said: "In this election Scotland has an opportunity like never before to make its voice heard at Westminster, but it is clear that the only party who can be that voice is the SNP.

"Polling shows that the SNP has become the national party of Scotland in all senses - leading in the polls across all areas and demographics of Scottish society."

She added: "Above all they are putting their trust in us to lock the Tories out of Downing Street.

"The real question for Ed Miliband and Jim Murphy is whether they would rather see David Cameron back in Downing Street than work with the SNP to lock the Tories out of government. That remains the big question hanging over the Labour campaign in Scotland."

Mr Salmond said: "Nicola Sturgeon is a first-class First Minister for Scotland and polling after Thursday's debate shows she was not just considered to have performed best by voters in Scotland but across the UK as well.

"The SNP always has and always will work to make Scotland stronger and better and a SNP vote in this election will be a vote to make Scotland's voice heard - loudly and clearly.

"Ours will be a voice for a new, better and more progressive politics at Westminster - for everyone across these islands."

Earlier today, Mr Salmond stepped up the pressure on Labour leader Ed Miliband with a fresh warning that he will find it difficult to avoid doing some form of deal with the SNP in the event of another hung parliament.

The former SNP leader said all parties would have to face up to the ''electorate's judgment'' after polling day on May 7 and that Mr Miliband had been ''foolish'' to rule out a coalition with the SNP - even though it was not really on the cards.

He told the BBC Radio 4 Today programme: ''I think that after the election every Westminster politician will have to come and face the reality of the electorate's judgement.

''There is no disrespect or disgrace in any politician coming to terms with the democratically expressed position of the electorate.

''All politicians, those of us who are lucky enough to be elected, chosen by the people, will try to do their best as they see it in the interests of the people who elected them.''

Lib Dem Sir Malcolm Bruce, who is standing down from the Gordon seat he has represented since 1983, said Scotland could find itself completely outside any government if it votes overwhelmingly for the SNP at the election.

Sir Malcolm claimed that people were waking up to the fact that a clean sweep of SNP MPs could give them no representation in government and so were thinking about voting tactically.

He told BBC Radio 4's Today programme: "I am meeting people who voted Yes, who are not voting for the SNP and feel it's maybe gone too far and we should move on.

"Secondly there wasn't a uniform vote across Scotland, there were very substantial variations.

"Here we are in the north east of Scotland, it was overwhelmingly against independence, certainly well over 60%.

"Therefore you cannot assume that the SNP will actually get enough votes automatically to hold or win seats in the north east or other parts of Scotland.

"We are finding that people are voting tactically because if you take these polls all the way through, Scotland could effectively put itself out of reach of being part of any government because we have elected nobody of the governing party, whatever it is.

"That's not good for Scotland."

Scottish Conservative leader Ruth Davidson has also be in the north where she joined party activists in Aberdeen city centre at a street stall and called for more apprenticeships for the offshore industry.

Ms Davidson said: ''I refuse to live in a country where we deem it acceptable for a young person to leave school and walk straight to the job queue. If just one young person has left education without the skills they need for a job, then we have failed.

''Our universities are doing more to address this, but we can do more. That's why the Scottish Conservatives are backing an extra 10,000 apprentices by the end of the decade.

''That's double the rate of increase proposed by the SNP and reflects the urgency of the situation we face."

Meanwhile, Scottish Labour leader Jim Murphy has been in Glasgow city centre, with a message about the choice facing voters on May 7.

A Lord Ashcroft poll published yesterday suggested Mr Murphy could lose his East Renfrewshire seat to the SNP but he argued that his party can offer progressive change in government.

He said: "In just 19 days time the people of Scotland face a simple choice. They can vote to change Scotland and the UK forever with the Labour party, or austerity max with the SNP's plans for full fiscal autonomy.

"Scottish Labour wants working class and middle class families to get a break after years of real pressure, and in too many cases, real pain.

"We have a costed plan, a positive programme based on Labour values and the people's priorities.

"We will reinforce our NHS with 1,000 more nurses and 500 more GPs. We will support all our young people to make the most of their lives, with a package of support for everyone, whether they are in work, at university or looking for a job.

"And our progressive taxes will make sure that those who have the most, pay their fair share towards rebuilding our economy."