RUTH Davidson has had quite a week. It started in the depths and ended on a high. Elections are always a rollercoaster, but this was a special kind of catapulting between extremes.

First, the pits. Tuesday’s Holyrood debate on the UK Government limiting child tax credits to two children per family was powerful, passionate and excruciating for the Scottish Tory leader. The focus was the “rape clause”, the exemption from the two-child cap that allows a woman who has had a later child through rape to apply for extra benefit – provided she names the child, and recounts her ordeal with third parties.

I cannot remember such rage in the chamber. Some Tories tried to kid themselves it was faux-outrage, “demonising” Ms Davidson because of the election. But no. The fury in Kezia Dugdale’s face as she read a letter from a rape victim, or the disgust in Sandra White’s voice, or the gutless way Tories read their phones instead of looking their critics in the eye – all real.

Many people asked why Ms Davidson, who prides herself on her opinions, didn’t disown the wretched thing. Besides any moral qualms, it looked politically mad to be on the wrong side of an argument with rape victims. Yet there she stood, soaking up abuse as MSPs waved copies of the eight-page form women must complete to help feed their kids, trying to argue it should be seen in the “context” of cutting the deficit. All 31 Tory MSPs, and only the 31 Tories, then voted to defend the measure. Why look such flint-hearted swine?

I suspect one answer arrives in Scotland today. Theresa May is in Aberdeenshire to discuss industrial strategy. So far, the rape clause hasn’t become an election issue in England in the way it has here. But if Ms Davidson, darling of quiz shows and the Westminster lobby, broke ranks to oppose it, the London media might suddenly take notice. There would be a risk of contagion, with Mrs May hounded over the clause in the same way as Ms Davidson.

One of Mrs May’s earliest supporters, Ms Davidson would not want that for her boss – or to be blamed for it. Because as her end of the week high-point showed, the prize for keeping the campaign on track is huge for the Tories. A new poll pointed to seven SNP scalps within reach on June 8, including that of deputy leader Angus Robertson.

First there are the local elections, but the Tories are excited here too. Starting from 2012’s low base, their vote share and councillor tally should shoot up. They also expect the SNP to oust Labour from most of its councils.

And as one Tory MSP told me, a good SNP result is also a good result for Tories. Because if Labour is crushed on May 4, the Tories become the last line of defence for the Union. That means the General Election becomes even more polarised, and voters who oppose a second referendum get funnelled into the blue column. Labour, meanwhile, would probably turn inward and start squabbling over Ms Dugdale’s future.

Yes, that suits the SNP, said the MSP, but it also brings new people to the Tories, and the Tories don’t need to win the election to stymie a referendum, they just need to burst the SNP’s bubble and paint Ms Sturgeon as a leader on the slide. Then it’s on to the 2021 Holyrood election and a reprise of the slogan that put the SNP into power in 2007 – time for change.

To come back in Scotland after a wilderness generation and turn those poll numbers into seats will not be easy. The Tories will need focus and an iron discipline. But think back to Ms Davidson and her regiment coldly refusing to back down over the rape clause. They may well be flint-hearted swine, but the SNP should be in no doubt, they’re on the march.