All economists know the theory of the second-best. This is the idea that a government action that would be sub-optimal in an ideal world can in fact improve welfare in a less than perfect one; for example, if there is monopoly, price controls which would be inefficient otherwise could make consumers better off.
Of course, in a perfect world politicians would tell the truth. But we don't live in that world, but instead in one in which the public are ill-informed and the media is biased. In such a world, a political lie might be justified on second-best grounds. Just as an otherwise undesirable price control can offset the sub-optimality of monopoly and so improve well-being, so an otherwise undesirable lie might offset untruth elsewhere and so move us closer to optimality.
This might not be true in the trivial context in which Sir Malcolm was speaking, but it could be so in economic policy. As Simon says, it "seems a hopeless task" that a political party could tell the truth and win elections.
If I were a politician, I would lie through my teeth about being tough on immigration and "scroungers", on the grounds that such dishonesty is necessary to get elected and implement better policy. Arguments to the contrary, it seems to me, are ones of fact rather than morality: is dishonesty really necessary or feasible? Isn't there a danger the lie will become the truth.
This, though, might not be the only example of the second-best in politics. Take our electoral system. This is biased against smaller parties, which seems undesirable. But one might argue that this bias offsets biases in favour of smaller parties - such as the excessive media exposure given to Farage or the egocentric bias which causes people to overweight their private beliefs. (I'm not sure I support this argument myself, but given the existence of the FPTP it must have some defence like it).
There is, though, a massive problem with such second-best thinking - it can justify pretty much anything. A general fighting a war in a good cause can use it to defend war crimes; the bombing of Dresden is still morally controversial today. This is especially so because the self-serving bias means we can always find a strong defence for our own behaviour. As Dan says, the defence that "in the system as it exists, we are forced to take measures to offset its weaknesses” is not a strong one.
In fact, the question I'm raising here is millennia-old: is it possible to remain virtuous whilst engaging with a morally comprised world? This isn't just a question for those considering a career in politics. It arises for pretty much any profession: law, journalism, banking or even - as Dan shows - academia.
There's a long tradition which answers this question in the negative: this was a big part of monasticism, an echo of which is found in Alasdair MacIntyre's claim that we should construct "local forms of community within which civility and the intellectual and moral life can be sustained." This might help explain why there is so much discontent among even well-paid professionals. Perhaps the old question deserves more explicit attention.