First past the post is a one-party system

If to be politically left-wing is the exact opposite of being right-wing then first past the post would pose no problem because each side would have a party to vote for. But the right is intrinsically less collectivist meaning that for it to vote for a single entity is more problematic and less likely to happen. So then the right loses votes because of a system which rewards only the winning candidate. To use first past the post is not offensive to the left in the same way that it is offensive to the right and so then it is a kind of subsidy to the left. Fptp is helpful to the left because left-wing voters do not mind voting for Labour in the same way that right-wing voters might object to voting for the Conservatives. First past the post gives the right only one (realistic) choice with which to oppose Labour which is a problem. The left don't mind so much being forced to vote for the Labour party to stop the right. The fptp system splits the right-wing vote more than it does for the left which means it is helpful to the left. To give the liberal right a better chance requires that a proportional system is used. Without pr the left will have an inherent advantage because the right are more offended by voting for a single party than the left are.

There is no (liberal) logic to first past the post

Voters tend to be opposed to the government otherwise elections would have no bearing on the state. Elections would be irrelevant if everyone is pro-state because each time the people would choose to move in the direction of more communism and a bigger state. It is because the people might oppose the government that elections are meaningful and that democracy tends to oppose communism. If democracy is not communist then more democracy would tend to weaken and reduce the influence of the state.

First past the post is less democratic than proportional representation because it offers the voter less (meaningful) choice. Because fptp is generally a two-party system voters must either waste their vote or vote tactically which means the people have much less power in a fptp election (and the government have more power). In a democracy the interests of the people (not the government) are expressed via an election. Elections are bad for the government which is why democracy is very close to anarchy and more democratic systems (such as pr) are more like anarchy (and worse for the government) than first past the post. Governments prefer for there to be very few elections and they prefer fptp to pr because it gives less power to the people.

Even in a democracy the people are not the government and so then what is good for the people (democracy) is not good for the government.

There is no good argument against first past the post

We are lucky if the state complies with the democratic process since to have an election is not entirely consistent with government. If we are a government then we do not necessarily seek the consent of the people ('it is inconsistent for the state to be democratic'). So democracy is a luxury and something we should be grateful for if we are given it by the state.

We can understand that the government doesn't like elections and would prefer to remain in power unchecked. Because the state has no obligation to hold elections this makes it very difficult to form a coherent argument in favour of proportional representation (over first-past-the-post). If we are lucky to have elections to begin with and democracy is a gift from the state then it is inconsistent to complain that the gift is not good enough. It is in the power of the state to subsidise itself by reducing the extent of its accountability to the people. The state can choose to subsidise itself by using fptp and there is nothing we can do about it. We can expect that the state would prefer to use first-past-the-post (over pr) unless the state is acting in the interests of the people and liberty. But since elections are granted by the government there is no strong argument against non-democracy and in favour of liberal proportionality.

Proportional representation is a moral obligation

For most people to be moral requires only that we do no crime... if we do not violate the rights of other people then this is sufficient to be a law-abiding person. The state however is slightly different in that it does have positive obligations. The most obvious of the obligations of the state is that it must be accountable to the people and hold elections. If the state assumes power without being democratically elected then it is acting in an immoral way and it is guilty of a crime. The state has a moral obligation to hold elections.

But not all elections are alike... if the state uses a winner-takes-all system then this gives an advantage to the larger political parties which can lead to an aristocratic state oligarchy surrounding the two main parties. To remedy this it is possible to have elections which (almost) guarantee proportionality. If the state doesn't use a proportional voting system then we can assume that the state is acting deliberately against the people and this (in a democracy) is a crime. The state has an obligation to hold democratic elections but it also has an obligation to make sure those elections are proportional. If the state doesn't provide proportional elections then it is illegitimate. The state has a moral obligation to make sure the means by which it is elected is proportional. The state is immoral if it doesn't use a system of proportional representation.

The Labour party should not fear the Tories

It would seem unlikely that either of the parties which benefit from the first-past-the-post system would be willing to remove it but there are stronger arguments for Labour doing so than the Tories. It is assumed that the Tories are the party of the establishment and that any lack of democracy is in their favour since any vacuum of power due to a lack of proportionality will be filled by the establishment. It is also consistent to imagine that when the first-past-the-post system was first set up the lack of proportionality was due in part out of a desire to protect the establishment... so then fptp is historically favourable to the Conservatives. Labour are a less ancient party than the Tories and so then they cannot be held responsible for the two-party fptp system. Whilst they are now the beneficiaries of fptp (along with the Tories) Labour are not responsible for its implementation.

We can assume that the Tory party will never be willing to replace fptp with pr because there will always be a small number of individuals in the Tory party who think that the total democracy of pr would be unacceptable and since only a small number of people at the top of the party need to hold this view it can be assumed that nothing will change. On the other hand things are not so clear-cut for Labour... they do not think of themselves as the party of the establishment and a more liberal system is not inconsistent with their beliefs. Labour are a populist party by definition (since they are left of centre) so for them democracy is not a contradiction. Labour would be less comfortable in the knowledge that they are in receipt of 'tactical' votes whereas for the Tories this is less offensive (the Tories don't mind being disliked). The only real reason for the Labour party to reject pr is that they fear upsetting the Tories. First-past-the-post is a Tory system but the Labour party should not fear the Tories and should be willing to replace fptp with the pr system. Upsetting the Tories should not be enough of a reason for Labour to reject pr. It shouldn't matter to Labour that the Tories dislike pr.

Proportional representation is liberal

There is no reason to reject democracy since the purpose of democracy is to legitimise the state. If the state seeks to remain legitimate then it must accept democracy and if the state doesn't use a proportional system then it can be more legitimate by changing the voting system. We expect the state to seek to be legitimate and so then it is consistent for the state to use pr. The only reason to use fptp is if the state is in conflict with the interests of the people (which it should not be). First-past-the-post is bad for liberal voters and since the purpose of democracy is to give a voice to the liberal electorate then fptp is inconsistent. We want liberals to be heard and represented and so then we must only use proportional representation. If we do not use pr then liberals have been excluded from the state which is wrong and a contradiction because the state must be liberal.

It is a crime to use first past the post

In any non-tyrannical society we assume that the innocent are given their freedom. This is what it means to live in a free society. This means that the government doesn't have the right to arbitrarily imprison individuals. But if there is not democracy then we can think of the people as being oppressed (since the state is then arbitrary) and we do not have a free society. To have a free society which is not anarchist requires democracy. But some forms of democracy are more liberal than others and it can be shown that proportional representation enables the voter to better express their preferences. If there is a winner-takes-all system voters must consider voting tactically since few parties have a realistic chance of success. So then we can see or it can be shown that pr is more liberal and at least there is no strong argument to show that it is illiberal. If there is no evidence to show that proportional representation is worse and more oppressive than fptp it should be adopted based on the principle of free-unless-guilty. If some people want to do something or to use a particular method of voting then in a liberal society they are permitted to do so unless there can be found evidence of harm or the risk of harm. Since there is no evidence of harm associated with pr (many peaceful countries use it) then in a liberal society it would be adopted. The rejection of a liberal voting system for which there is no evidence of harm demonstrates an illiberal society. Since there is no evidence against pr any country which refuses to adopt it is illiberal and totalitarian.