With the article the preceding an adjective, the resulted phrase is usually treated as a plural noun. Examples are “the poor”, “the free”, “the brave”, “the wise”, etc..
It seems that the phrase “the left” or “the Left” (when it refers to political groups) is an exception. Sometimes it is followed by singular verbs, sometimes it is followed by plural verbs. My question is:
What is so special about the phrase “the left”? When it is used as a singular noun, does it somehow convey a subtly different meaning than it being used as a plural noun?
The general rule is this: when the people being referred to are a coherent unit then the singular is used: the Catholic Church, the Liberal party, the Finance committee can all be referred to in the singular, especially when you are talking about collective actions or attributes. The organization is the singular entity being referred to. So:
The Church promotes belief in God
The Liberal party supports tax reform
The Finance committee approves the budget
However for many of the above the plural can also be used, especially when the action is by individual members rather than the group as a whole. And there are exceptions.
The Finance committee believe that the current budget is the best.
On the other hand the poor, the brave etc. are not any kind of coherent unit, they are just a number of people. Therefore they will always be plural.
The poor spend most of their money on housing
The brave live on in our memories
Using singular for ‘the left’ implies that you are treating them as an organised unit. Exactly the same rules and argument applies to ‘the right’ and ‘the centre’.