Labour has been given a boost after backing from an unexpected source - the leader of the Tory Party in Scotland.
Douglas Ross urged voters to "do what's best for the country" and vote Labour if they're best placed to beat the SNP.
The move has sparked anger across the Tory hierarchy, with the Westminster party saying tactical voting is "emphatically not the view of the Conservative Party".
In a bombshell interview with The Sunday Telegraph, Mr Ross said: "The public know how to tactically vote in Scotland.
"I will always encourage Scottish Conservative voters to vote Scottish Conservatives, but I think generally the public can see and they want the parties to accept that where there is a strongest candidate to beat the SNP you get behind that candidate.
"If parties maybe look beyond their own narrow party agenda and do what's best for the country and for me as Scottish Conservative leader what would be best is if we see this grip that the SNP have on Scotland at the moment is loosened."
However party bosses in Westminster were far from impressed.
A spokesman said: "This is emphatically not the view of the Conservative Party," he said.
"We want people to vote for Conservative candidates wherever they are standing as that's the best way to keep Labour and the SNP out."
It comes as the SNP battles the worst crisis in its history following the arrest of former chief executive Peter Murrell in the investigation into the spending of around £600,000 earmarked for an independence campaign.
In her first public comments since the arrest on Wednesday, Ms Sturgeon said she would "fully cooperate" with the police if they ask to interview her.
"I haven't, but I will fully cooperate with the police as and when they request that, if indeed they do," she told reporters outside her Glasgow home, when taking questions after giving a brief statement.
SNP president Mike Russell told The Herald newspaper the party was in the "most challenging crisis we've ever faced" during his 50 years involved.
Labour and the Lib Dems are expected to be the main challengers to SNP candidates in the next general election.
