This is of course especially true during general election campaigns.
Left politicians often have to fight their way through a miasma of misrepresentation, biased reporting and ill-informed criticism. But, even if they beat the odds and win an election, they will find that being in government can mean that, in some ways, the going gets even tougher.
They will suddenly find themselves being criticised not just for what they would like to do, but for what they actually do in government.
There is no point in complaining about this - just get used to it, it comes with the territory.
In a democracy, it is the job of the forces opposed to the government to hold it to account and to make reasoned (and sometimes not so reasoned) criticisms of anything the government may do or propose.
After her success in forming a Government, Jacinda Ardern will find that everything she (and her Government) does and says will be subject to attack - sometimes, in the case of the left's habitual critics, well in advance of their being able to identify anything worth attacking.
And a new government, especially one that has been out of office for some time, can sometimes find it difficult to move out of campaigning mode.
It can become, if it is not careful, immediately focused on winning the next election, rather than sticking to the job in hand and using the election victory it has just won to do what it was elected to do.
It could, as a consequence, become preoccupied with avoiding giving offence and with warding off criticism so that it dare not try anything new.
Opponents and critics can come to seem more important than the government's own supporters as determinants of what the government does.
Of course, it makes sense not to alienate unnecessarily those who disagree - and it is of course always possible that critics can be disarmed, even change their minds.
But the danger is that excessive attention to critics can knock the government off course, even before it really gets started.
The best antidote is for the government not to lose its nerve - or its heart - and not to become shackled by the orthodox thinking urged on it by its defeated opponents and their supporters.
A new government should trust the good sense of those who elected it in the first place. They voted for the government and its component parts (and this is especially true of the 2017 election) because they wanted to see some painfully obvious and damaging issues addressed.
They recognised that urgent action was needed on child poverty, on growing inequality and social dislocation, on homelessness, on the neglect of mental health and on climate change.
They voted for the new coalition Government in the hope and belief that it would deliver on those - and other - issues.
The best chance of retaining their support and carrying it into the next election in 2020 is to meet those expectations.
The critics, in other words, are always with us - not, of course, to be disregarded, but not those either whose views and (often irrational) fears should provide the Government's motivation and momentum.
The people who really matter to the new Government are those who hope for courage and new thinking - and a determination to make a fresh start.
They are where the new Government's focus must be.
That way lies not only real achievements in the here and now, but also the best chance, through keeping faith with its supporters, of winning the next election - and the one after that - as well.
