Although Mr Trimble's Ulster Unionists have always been the largest Protestant party, an examination of past elections shows that on certain occasions large numbers of its supporters have switched to Mr Paisley's Democratic Unionists.
The two parties are generally at odds, following the old precept that the battle between rivals can often be more bitter than that between outright opponents. But the figures show that many at grassroots level are floating voters who regularly switch votes between the two.
The key question in Friday's vote is whether they will stick with the Trimble camp or, in the less frequent but established pattern, go with Paisley. There has never been an exact precedent for this referendum, but past performance sheds some light on the question.
Northern Ireland has gone through 22 elections in the last quarter of a century. Mr Paisley's worst performances come in Westminster elections where he averages 15 per cent.
This is because the sitting Unionist MPs are mostly members of the Ulster Unionists. In a number of cases Mr Paisley is politically unable to run against these, since a split Protestant vote could allow a nationalist to take the seat.
The absence of DUP candidates in such seats keeps the Paisley vote unusually low.
A different pattern can be seen in council elections where the proportional representation voting system removes the danger of split Unionist votes. In these contests Mr Paisley takes an average of 20 per cent of the vote, a significant increase on his Westminster showing, though still behind Ulster Unionist levels.
This pattern is, however, dramatically reversed in European elections, which, ever since the 1970s, have represented a recurring personal triumph for Mr Paisley. In these contests he outstrips Mr Trimble's party, zooming ahead to an average vote of 31 per cent and taking a clear majority of Unionist votes.
These polls are regarded as "beauty contests," being the only occasions when all ordinary Protestants can vote for Mr Paisley personally rather than voting for his party.
Although the DUP leader himself lays much stress on these particular contests, most commentators agree that European votes are much less important than elections to Westminster.
The significance of these European polls is that traditional Ulster Unionist party supporters feel free to transfer their allegiance to the DUP leader. His hope is that they will do so in large numbers on Friday, enabling him to claim that a majority of Unionists are against the agreement and have given him a mandate to wreck it.