Large sums are made by authors, trainers and self-appointed gurus claiming to know what it takes to be or become a leaders. Politicians, at least elected ones, have to be leaders. They set themselves up to be followed by thousands, after all. Their success at the ballot box may be merited by their talent; it may also reflect their brass neck in putting themselves forward and/or the brand to which they attach themselves. Their staying power or the hindsight of history produce the evidence as to the validity of their selection. Another factor may be worthwhile including in this judgement: the greater good.
I suppose that most entering politics genuinely want to do something worthwhile on behalf of their community. What is clear, though, is that the power which popular support offers to those elected can affect the latter in different ways. To some, it increases their sense of responsibility and drives their energies. To others, it feeds ambition and creates a sense of entitlement or grandiosity which can actually prevent them from being good leaders. My suggestion is that those who maintain their original or even an enhanced desire for the greater good are likely to be far better leaders than those others who let it all go to their heads. It does not take high office for this leadership to manifest itself. There are politicians who act as the conscience of their peers or content themselves with achieving social good. These can be seen as leaders just as much as those in charge of departments of State - and some of the latter should not be where they are, based on this criterion.
Those who accept the votes and responsibility without delivering on their original ethos are not leaders but failures. We know who they are, even if they do not; but too late. The trick is to be able to identify at the selection or election stage who will become which. This is not so easy but integrity should overrule ambition, perhaps, as a yardstick.