SO far, two Members of Parliament have defected from the SNP to the new Alba Party. The avalanche of defections some were expecting seems to have slowed to a trickle, but now is nevertheless a good time to consider the role of parties, party loyalty and party discipline in a parliamentary democracy.

In a free society there are diversities of interest and opinion. Decent and rational people may legitimately differ in their priorities and policy preferences, and in their estimation of the relative merits of various potential leaders.

The existence of parties – famously defined by the 18th century statesman Edmund Burke as “a body of men united for promoting by their joint endeavours the national interest, upon some particular principle in which they are all agree” – reflects the reality of that difference.

Parties speak for part of the people. Some parties define themselves in terms of ideology: as liberals, socialists or conservatives. Others define themselves in terms of interest: as the party of workers, a farmers’ party, or a party of a particular ethnic, religious or national minority. Even the so-called “catch all” parties that aim for mass appeal cannot speak for everyone.

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Since parties necessarily reflect the disagreements and divisions of society, there must always – in a democracy – be a genuine choice of parties. The extent of that choice varies, however. In countries with First Past the Post electoral systems there might be two major parties, each competing to win a majority of seats and form the next Government, while the other minor parties have little hope of achieving a breakthrough that would bring them into power. In countries with highly proportional electoral systems, one finds a broader array of parties; with no one party expecting to win a majority, coalition governments are the norm.

Either way, political parties are the links in the great chain of delegation and accountability extending from the ordinary voter to the head of government. They select candidates for elective office, mobilise support, and reduce otherwise impossibly complex political choices to a manageable number of options from which voters can sensibly choose. They enable public demands to be articulated and prioritised, channelled into a credible manifesto, and then transformed into a coherent policy agenda and legislative programme. As they all stand together, so must they all fall together, voters can collectively punish a party at the next election.

Without party loyalty and party discipline, this chain breaks. Parliamentarians cease to be the instruments through which public demands are transformed into public policy. They become instead independent brokers in an oligarchic, unaccountable “politics of notables”.

That is why many have argued that members who leave their party should resign and stand in a by-election. While personal or local factors sometimes make a marginal difference, most people usually vote on party lines. Members are primarily elected on a party label; they do not merely represent their own views, but also those of the party who has backed them. Members who quit their party therefore forfeit much, if not all, of their legitimacy.

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In the UK, this rule has relied on unwritten norms and never been enforced. Many Commonwealth countries constitutions do however contain “anti-defection clauses” expressly penalising “floor crossing”. Examples include countries as diverse as South Africa, India, Bangladesh, Fiji, Samoa and Belize.

While party discipline is important, so too are the maverick backbencher, the critical select committee chair, and principled member willing to take a stand against their own party leader. Without these, parliament is a much poorer, weaker institution. Bangladesh has an anti-defection clause so draconian that voting against the party whip, or even abstaining, results in a member losing his or her seat. Such extremes would destroy the freedom of parliamentarians, undermine all notions of parliamentary accountability, and abolish Parliament as a genuine place of deliberation and decision.

Even defections can be useful. They enable new parties to be formed, opening new avenues that might be blocked by the existing parties. The formation of the SDP in 1981, and the Independent Group in 2019, are two examples of defections that sought to change the political landscape. Ultimately the people must decide.

The question is when the people should decide. Perhaps a year is a reasonable period. A written constitution might provide that member splitting from their party should have a year’s grace. At the end of that – if they have not already faced a general election, or resigned and won a by-election, their seat should be declared vacant. That arrangement would at least give some elasticity and flexibility, while maintaining the primacy of the party as the conduit between people and Government.

Chris Hanlon, head of SNP’s Policy Development is the guest on the TNT show on Wednesday