This inherent difficulty is the lesson from past referendums and it is one that is well understood by key members of the NO campaign, including Tony Abbott, who played a key role in fashioning the anti-republican slogans in 1999. One of the key messages was "If you don't know vote NO".

Once again a key tactic of the "no" campaign will be to introduce uncertainty into the minds of those citizens who cast a postal "vote" in this voluntary plebiscite. This can be done in several ways.

The first will be to raise questions about the type of same-sex marriage legislation which is envisaged. This tactic has already begun with the request that the draft bill legalising same-sex marriage be published prior to voting beginning. This is a carbon copy of the argument that a republican plebiscite can only be meaningful if the ultimate type of republic is decided before such a plebiscite is held.

In this instance this request is difficult to comply with, if not meaningless, because the precise bill will follow parliamentary debate and will include amendments negotiated in the House and the Senate during the parliamentary process. But it is a request which the "yes" campaign should bear in mind as the more certainty during a campaign like this the better it will be for its campaign.

But the major way in which uncertainty can be introduced is by broadening the discussion so much that the possibility for uncertainty in the mind of the voter is greater.

The "yes" campaign will win a narrowly focussed campaign on the merits or otherwise of expanding the definition of marriage to include same-sex marriage. It will win a campaign based around personal human rights, loving relationships and stable partnerships. These arguments make for a campaign narrative which can be illustrated with so many poignant human stories and is difficult to rebut on its own terms. That is the case so far and explains the popularity of same-sex marriage in opinion polls.

The "no" campaign wants to change the terms of the debate. In election-speak it wants to fight on its own ground not ground chosen by the "yes" campaign.

That is what Abbott was attempting to do in his immediate foray into the campaign. It was a political campaigning master class. His aim was to frame the debate as not about marriage as such but about bigger and more controversial topics, including freedom of religion, freedom of speech and political correctness. The more red herrings the better.

Some other "no" campaigners have emphasised those elements of this approach which suit them. Not surprisingly church leaders like Anthony Fisher, Catholic Archbishop of Sydney, are emphasising freedom of religion. Such an approach probably resonates more with an American than an Australian audience but it still could have some force among religious believers.

Other elements of the "no" campaign, like the Australian Christian Lobby, will build on Abbott's identification of opposition to political correctness as a key issue. Such an approach will broaden the debate into the area of Safe Schools and gender politics more generally.

The wider the debate the broader the "no" coalition may become. A broad coalition can camouflage the inherent weaknesses of some of the campaigners, such as the churches. Victory for a campaign led by the churches alone would fly in the face of the conventional wisdom, magnified by the recent publication of the 2016 Census results, about their failing strength and damaged brand.

But the main benefit of a broad debate is that it is easier to muddy the waters. This is the political consequence of such a strategy. The muddier the water the better the environment for a successful "no" campaign, because building uncertainty is a key aim of all negative campaigns against change. In such an environment advice to vote "no" if you are at all uncertain can have real potency.

To be successful the "yes" campaign must attempt to counter this thrust from the opposition. One way is to run its own positive campaign without regard to what the "no" campaign is saying. It must play to its own strengths which are extensive. They include focusing on the benefits of such change and the absence of any evidence of harm. The "yes" campaign will also draw on international examples from comparable countries of the trouble-free introduction of same-sex marriage.

But it also must try to lessen any uncertainty by making its case for change as precise, predictable and as focused as possible. Each element of the broad "no" campaign must be addressed by those best-placed to respond. The argument about dangers to religious freedom must be addressed by respected members of faith communities. The arguments about Safe Schools must be taken up by respected educationists. And so on.

There is already high emotion, nastiness and offence given and taken in this campaign. But ultimately these regrettable aspects of campaigning will not decide the outcome.

What both sides need to win is hard-headed thinking and hard-nosed strategy and tactics. The "no" side may be the under-dog but will spare no effort to defeat this push for change. In their favour is the fact that Australia is a conservative country and negative campaigns are always easier to run and win than positive ones.

John Warhurst is an Emeritus Professor of Political Science at the Australian National University.