"If we start a campaign, people will flock to us." Experience around the country makes it very clear that this is wishful thinking.
Never assume people will flock to you. In fact, some people will join in, but also some
people you were counting on won't come through as expected. You should not plan
on any more support than you have firm commitments for.
"We are right, therefore we will win." Not so. You will win only if you convince a majority of the voters that you are right,
and if you have the strength to get on the ballot. Note that once PR and IRV have made
it to the ballot in modern times in the USA, only once have we won, while losing four
times.
"We can win PR this year." (Or "next year") That might be true, but it usually isn't, especially if you have a new, small organization.
This is only true if you've lain a strong foundation.
"Even if we don't get on the ballot, we will educate people and energize our activists." You will educate people, and in the short run you will energize your activists. But after
you fail to make the ballot, you will have discouraged activists, and you will have lost
credibility with "opinion makers" and your activists. Failing even to make the ballot is
clearly a long term negative, and is highly discouraged.
"Even if we do lose once we get on the ballot, we'll have opened up the discussion." Again, this is true, but fails to take into account the long term damage caused
by losing. You can afford to lose once, perhaps, if you don't lose too badly, but if you lose
twice you have probably lost all credibility with opinion makers and your volunteers.
Losing is usually bad, when you eventually need to win.