LIKE dogs, Scottish budgets are not just for Christmas. The draft tax and spending plans for 2018/19 announced by Derek Mackay on Thursday were only the start of a rollercoaster that will go on for two more months.

Holyrood’s committees will scrutinise, ponder and opine on the Finance Secretary’s numbers, and the SNP will continue horse-trading to muster the voter to pass a final, rejigged Budget bill in February.

The Greens love a good haggle, and are already demanding £150m for councils for their support.

As the BBC’s Brian Taylor notes, Mr Mackay might find some cash by not raising the threshold for the higher rate of income tax, which is how he met a Green demand for £160m for councils earlier this year.

A lot of this is political Christmas panto, of course. The Greens want to claim they’re leading the SNP by the nose down a radical path.

There is weary eye-rolling at this within the SNP government, where ministers know the Greens need their their moment in the sun.

However there’s also a definite resistance to being pushed around.

One government source close to the Budget told me they had been ready for a failure of talks with the Greens and a snap election. There is little appetite for appeasing them more. Besides, they added, the Greens aren’t the only show in town - the lean and hungry LibDems are always looking to cut a deal.

Labour are nowhere in all this. Richard Leonard, a month as leader without naming a shadow cabinet, doesn’t seem to know what he’d do in budget talks, bar declaim at folk.

Although they’re even more remote from the process, the Scottish Conservatives think they’re the big winners from the Budget.

A few hours after Mr Mackay set out his plans, the Scotland Office held its annual Christmas drinks party near Holyrood. Jubilant Tories thought his 38p a week tax cut for the low paid was so derisory voters would barely notice it, and certainly wouldn’t thank him for it.

A worse gaffe and insult than Gordon Brown’s infamous 75p a week pension rise when he was Chancellor in 1999, said one MSP.

In contrast, the person continued, the voters seeing tax rises would definitely remember their “Nat tax”.

The Tories opened up a second line of attack yesterday, using woeful revised growth forecasts to claim a “Sturgeon slump” was round the corner, as SNP policies cratered the economy.

Comparing the government’s forecasts from February with the latest ones from the Scottish Fiscal Commission, they said tax revenues by 2022 would be £2bn south of where ministers expected.

The SNP’s higher taxes were creating a vicious circle, they suggested. Higher taxes relative to England drove away jobs, which hurt the economy, depressed tax revenues, and forced the SNP to raise taxes more to plug the gap.

This was ridiculously simplistic, even for politicians. On Friday, the same Scottish Fiscal Commission, the independent scrutiny body that crunches the government’s Budget numbers, gave a handy briefing on their “subdued” growth forecasts.

They hadn’t modelled the impact of higher income tax on the macro-economy in 2018/19 because they didn’t think there would be one.

That £164m hike spread across 1.1m people was just too small.

Instead, the Commission said the biggest problems for the economy ran far deeper - low productivity, which has been stuck in a rut since the 2008 crash, and demography.

There are just not enough working age people paying taxes, and an ever expanding elderly population increasing the demand on services.

Brexit will worsen the problem.

A tighter, post-EU immigration regime will stymie the rise in 16 to 64 year olds Scotland needs to expand its economy. With stagnant productivity also a problem south of the border, the Commission said the healthier GDP outlook in the rest of the UK was almost entirely down to its historically faster population growth - not lower income tax.

So consider the next Holyrood election in 2021. By then, if the Tories are still in power at Westminster, Scotland will have had five years of income tax divergence from the rest of the UK, with the revenue raised used to help protect public services.

The SNP will hope this difference is seen as a defining virtue, the Tories a Nationalist millstone.

But what is the Tories’ doorstep pitch to voters? Elect us and we’ll undo those years of difference and re-align the Scottish tax system with the one in the rest of the UK?

That would come at an eye-watering cost. People may not be vocal in their appreciation of public services, but they sure hate it when they’re taken away.

The SNP would try to paint the Tories as vandals, as Tweedy barbarians at the gate intent on dismantling the NHS and councils to give tax cuts to their rich chums.

That is a battleground Nicola Sturgeon would be more than happy to meet Ruth Davidson on.

It may be that, years down the line, Scotland’s different tax rates become a factor in population numbers and hence economic growth, but we’re not there yet.

Much bigger forces are at work, and no government, of whatever hue, can wave a wand and fix them.