With the support of the Scottish Greens, the SNP will win the vote calling for a second referendum on Scottish independence.
They will then claim that the UK government must not stand in the way of the democratically expressed will of the Scottish Parliament.
But the opposition parties - the Conservatives, Labour and the Liberal Democrats - all believe that the will of the parliament does not match the will of the people.
Scottish voters don't want another referendum, they argue, and they are convinced public opinion is on their side.
It used to be received wisdom in Scottish politics that if Westminster tried to deny a Scottish referendum that could easily backfire and stir up support for independence.
But the Unionist parties are confident there is no great public demand for another vote - other than among people who are already committed nationalists.
Opinion polls suggest people are split fairly evenly, about 50/50, on whether they want another vote.
And it tends to be yes supporters who say yes to another referendum. No voters who say no, not now.
The great divide in Scottish politics over the question of independence may only be solidified by this current debate over whether to re-run the 2014 referendum.
First Minister Nicola Sturgeon will argue that this is now a constitutional struggle in which the Scottish Parliament is battling with Westminster.
Tories say she is just playing a well rehearsed political game - one in which she puts forward a proposal she knows will be rejected by Westminster and then responds with righteous indignation.
And we are about to see that happen once again.
Ms Sturgeon will soon send a letter to 10 Downing Street demanding another referendum.
The Prime Minster Theresa May will reply to say they are not prepared to talk about it at this time, pressing the point that it is more important to pull together and get the best Brexit deal for the whole of the UK.
The big question, the big calculation for both sides, is how many Scottish voters will then share the SNP's inevitable outrage about being denied another referendum.
Some will. Others will breathe a huge sigh of relief that they don't have to live through another campaign. At least not yet.
The result of the vote in the Scottish parliament this evening is not in any doubt.