Today was a big moment for First Minister John Swinney as he unveiled his party's general election manifesto with just over two weeks to go before polling day.

Mr Swinney took up the role of SNP leader on May 7 succeeding Humza Yousaf who stepped down following the collapse of the Bute House power sharing arrangement with the Scottish Greens and had sat as a backbench MSP in Holyrood since Mr Yousaf's predecessor Nicola Sturgeon resigned in April last year.

To say it's been a torrid time for the party is something of a significant understatement.

Mr Swinney himself alluded its troubles when he was questioned in Edinburgh by reporters after his speech who asked him why so many Scots who voted at the SNP at previous elections were now considering backing Labour on July 4.

READ MORE: Swinney insists Holyrood result a mandate for indyref

Indeed, that polling trend appeared to have been confirmed last night with a mega poll by Ipsos Mori suggesting the SNP could be down to 15 seats (from 48 in 2019) while Labour would win 28, up from two currently.

Mr Swinney and the SNP will be hoping key messages in his speech today and proposals outlined in the manifesto will persuade those voters who have switched to Labour to come back and support the SNP again, helping to lift the party out of the doldrums it currently finds itself in.

That is the hope. But can it be fulfilled?

Well we'll have some idea of any shifts back to the SNP when the next set of polling results are published. They will help to give a sense of how voters have responded to the policies set out in the manifesto and to Mr Swinney's statements on issues such as austerity, Brexit, the cost of living and on independence.

READ MORE: Poll: SNP faces wipeout in Glasgow and losses to Labour

However, the SNP face a huge battle in its drive to win back former voters.

Political scientists often talk about political momentum in a campaign, the process which sees one party gaining support sometimes so fast it appears impossible to stop.

The SNP began to get this momentum in the run up to the independence referendum in September 2014 and by the following May's general election the yellow juggernaut could sweep aside any obstacle in its way.

Its momentum began to slow towards the end of Ms Sturgeon's time in power before it came to a crashing halt with her resignation, the bitter SNP leadership campaign which followed and then the escalation of the police probe into SNP finances.

And once lost it needs a huge effort for momentum to get going again, something dramatic to ignite the whole process of movement.

So what did the SNP offer voters and in particular its former voters today in its bid to get on the front foot?

Mr Swinney made statements of intent, heard before, that the party wants to hold a second independence referendum and a suggestion it still had a mandate for one from the 2021 Holyrood election (conveniently ignoring the Supreme Court ruling that the Scottish Parliament does not have the power to hold a second independence referendum).

Many of the proposals in the document and spoken about by Mr Swinney in his address were restating existing policies - eg removing nuclear weapons from the Clyde, the end to the two child benefit cap, abolishing the House of Lords and a commitment to rejoin the EU should Scotland become independent.

There was also a pledge of existing policy to demand an immediate ceasefire in Gaza and an end to the UK scheme to send illegal migrants to Rwanda as well as a promise to table a Bill at Westminster that will end any risk of NHS privatisation (also previously announced).

The party's big new plan - trailed in advance to the press last night - was to ask whichever party wins the general election to commit to spending at least £10 billion extra annually on health across the UK with the sum seeing around £1 billion granted to Scotland through Barnett consequentials which Mr Swinney said could bolster the NHS north of the border.

It is certainly a headline grabbing idea, but will it make any difference to the SNP's fortunes in the election?

In truth, barring any major upset for Labour, and with the SNP continuing to face immense challenges, that is highly doubtful.