Mr Darling has written to thousands of voters in Conservative-held seats in an attempt to persuade them to give their second vote to Scottish Labour, insisting the party had changed since Mr Sarwar took over as leader two months ago.
Despite working closely with the Tories in the successful Better Together campaign to keep Scotland in the UK in the 2014 vote, a move that many believe has damaged Labour, Mr Darling has now made a direct appeal to Conservative voters to ditch their party on the peach ballot paper and vote Labour.
Both Labour and Tory leaders have admitted they are fighting for second place in the Holyrood elections as opinion polls point to the return of an SNP government.
In a letter targeted at those living in Tory constituencies, Mr Darling claimed that "voting Conservative with your second vote won't help stop the SNP”.
He writes that while the No campaign “won the referendum”, the SNP “has continued its divisive campaign, which is so damaging to our country”.
Mr Darling adds: “This Thursday, you have two votes in the Scottish Parliament election. Every vote the SNP receives it will claim is a vote for another referendum – even though we know this is not a priority for most Scots.
"We can take the threat of another referendum off the table by preventing an SNP majority in the Parliament.
“The only way to stop an SNP majority is by voting Scottish Labour with your second vote – on the peach ballot paper.”
However, the Scottish Conservatives have insisted they are the best-placed party to stop a second independence referendum, and have similarly called for No voters of all political persuasions to “lend” them their second vote.
Ruth Davidson has also today written an open letter to pro-UK voters urging them to put aside party allegiances and vote tactically for the Conservatives with their peach ballot paper.
In a message sent to hundreds of thousands of Scots, the former party leader said the election came down to “a simple decision between another referendum or Scotland’s recovery”.
She also pointed to opinion polls since March, which have shown the Scottish Conservatives with a lead on the list vote.
Despite the polls, which have shown Labour potentially losing around four MSPs, Mr Darling said only voting for his party would “help stop the SNP”.
He said: “You may not have voted Labour in the past, or you may have left us in recent years, but under Anas Sarwar’s leadership we have changed.
“Anas Sarwar shares your priorities. His uplifting election campaign has focused on a national recovery plan for Scotland, not re-running the arguments from 2014.
"He opposes independence and he opposes a second referendum. He will put the NHS, jobs, education, climate and communities first.
"With a strong group of Labour MSPs behind him, he can spend the next five years standing up for a united Scotland and standing up to the SNP, keeping Nicola Sturgeon focused on recovery – not a referendum.”