The normally polite Lib Dem boss turned some very personal fire on the Tory leader as he described him as “part of the problem” the Conservatives have in Scotland.

At a press conference for the launch of the Lib Dem manifesto Rennie claimed his party was picking up support amongst soft Conservative voters because “there is a darker edge to the Conservative campaign this time and they are repelled by that.”

Rennie said that change was due to Douglas Ross being in charge of the Scottish Tories.

He said: “My politics and his are different. I think there’s a realisation amongst the public that he’s got different priorities than the kind of centre-ground Ruth Davidson and I think it is making a quite a difference.”

“The problem that the Conservatives have in Scotland is that they have had quite an ugly, poisonous period. They were quite right to challenge the Government on some things in the last year, but some of the ways that was prosecuted was quite was quite dark.”

Rennie added that on issues like the vote of no confidence in Nicola Sturgeon over the Salmond inquiry “there was a lack of reason in all that and I think Douglas is part of the problem.”

The Lib Dem leader drove home the attack by saying the Moray MP’s 2018 comments on gypsy travellers, when Ross said in a quick-fire interview that he wished he could see “tougher enforcement against Gypsies and Travellers”, were very revealing.

Rennie said: “I think that will hang over him from for some time because it was an immediate reaction to something, and often an immediate reaction can tell you an awful lot about someone’s character.”

He added: “Douglas himself is a bit darker than Ruth. Ruth was a bubbly bright individual. Douglas’s approach on things like gypsy travellers and on things like supporting Boris to become Prime Minister, he’s not got that kind of rebellious streak that Ruth had.”

The Tories, Rennie said, had policies to the right of Ruth Davidson. “ For all those reasons, I think there’s a darker image and a darker positioning amongst the Conservatives than there was previously.”

“Even standing on a jeep with a black mask on just looked a bit darker.”