Only those with little knowledge of the Scottish National Party (SNP) imagined that a large contingent of SNP MPs would lead to chaos and mayhem in the Palace of Westminster. The SNP’s landslide - winning 56 of Scotland’s 59 seats with 50% of the vote - was built on eight years of SNP Government as well as the 2014 referendum. The SNP advance was not won on the promise of endless confrontation or on a hard left prospectus.

The background to the SNP landslide in May lies in the politics of devolution. Prior to devolution the party was a marginal force in Westminster politics. Devolution with its more proportional electoral system created new opportunities for the SNP. The SNP became the main opposition party in Scotland for the first time.

But without a record of government, the SNP was vulnerable to attacks based on H.P. Lovecraft’s dictum that the oldest and strongest emotion is fear and the oldest and strongest kind of fear is fear of the unknown. An SNP Government was the great unknown. However as the main opposition party the SNP benefitted when voters became disenchanted with Labour. It was a combination of a Government losing an election and an Opposition winning.

The SNP leadership believed (or hoped) that this would translate into support for independence but after four years in office support for independence was static with only a quarter of voters choosing independence between the status quo, more powers and independence.

In 2011, the combination of its reputation for competence and the low salience of Scottish independence ensured that the SNP bucked the electoral system and gave it an overall majority. Little attention had been paid to its manifesto commitment to a referendum on independence as few imagined that the SNP would command an overall majority. But the overall majority ensured that a referendum was firmly on the agenda.

Support for independence grew over the forty months after the 2011 elections but not enough to win the referendum. But 30 per cent of those who had voted Labour in 2011 supported independence in the referendum. This loosened attachment to the Labour Party. A key part of this was the anti-Conservative message that dominated the campaign for independence. The referendum was a staging post for Labour voters to move to the SNP. Labour’s association with the Conservatives in the referendum had the same effect on Labour support as joining the coalition with the Conservatives had had on the Liberal Democrats.

The impact of the referendum on the SNP’s prospects at the UK general election only eight months later was considerable but more complex than simply reflecting an increase in support for independence. Support for independence had been mobilized and relatively easily channeled into support for the SNP. A couple of years before, opponent's of the SNP had assumed that a decisive victory for the union would damage the SNP and undermine its support at the UK general election in 2015 and the Scottish elections in 2016. The referendum proved a Pyrrhic victory for Labour and the SNP’s defeat the opposite.

The SNP appeal in 2015 was, as in its previous electoral advances, based on reassuring messages. Labour and SNP had competed to be the anti-Tory party in Scotland and the SNP’s unambiguous anti-austerity message was clear. The SNP insisted that the May 2015 general election was not a re-run of the referendum and that it was not seeking a mandate to hold another referendum. It proposed to share government with Labour reminding Scottish voters of its record of competence in Holyrood and its willingness to work with others to oppose the Conservatives. Labour’s refusal to work with the SNP reminded voters of Labour working with the Conservatives under the banner of Better Together in the referendum. This helped the SNP portray itself as the main opposition to the Conservatives in Scotland.

The SNP’s new MPs will try to avoid anything that might undermine the SNP Government in Edinburgh. Disruptive Parliamentary behaviour would undermine its image as a party of government and aim to be seen as the natural party of Scottish Government. The SNP will strenuously oppose the Government. But the SNP and Scottish public know well that oppositions do not govern and the SNP’s inability to block Conservative Government policy will provide ammunition in making the case for independence.