The Labour MSP’s apparent jab came as he published a video ostensibly celebrating 50 years since Labour's Programme 1973.
The programme, which Leonard said was “one of the finest documents containing socialist ideas ever produced by the Labour Party”, came ahead of the double-election year of 1974.
Labour, narrowly, won both of those elections and Leonard said the 1973 document stood in direct contradiction to “the theory that Labour cannot be elected on a left-wing programme”.
Quoting its opening lines, Leonard said: “We are a democratic socialist party and proud of it. We put the principles of democracy and socialism above consideration of privilege and market economics.”
OTD in 1973: Labour's radical and election-winning programme was published. pic.twitter.com/bqihIVkuBZ
— Richard Leonard (@LabourRichard) June 7, 2023
The video is one of a number to have come out of the former Scottish Labour leader’s office in recent months.
Another, titled “A Leader Called Keir”, was published in February and was also viewed as a broadside at the UK party leader.
In that clip, Leonard said: “With the Tories again attacking our trade unions, in the middle of this latest cost of living crisis, it is no good us supporting workers’ rights without also supporting workers when they exercise those rights.”
At Holyrood, the campaign is widely seen as an attack on UK Labour chief Keir Starmer’s leadership of the party, which has seen him attempt to distance himself from striking workers and move away from the left-wing pledges which he made during his bid to take control over the party.
Among the “10 pledges” which Starmer made to Labour members but has since dropped are tax rises for the highest earners and plans to renationalise some key industries, including water and rail.
Leonard was seen as a more left-wing candidate when he beat Anas Sarwar to the post in the 2017 Scottish Labour leadership race.
Sarwar, a more centrist candidate, took over after Leonard was ousted – apparently following direct intervention from Starmer.
The UK Labour leader has recently been exercising an iron grip on party selection processes, seemingly in an effort to remove more left-wing members from his party and install centrist loyalists in key seats.
A Holyrood source suggested to The National that there was unrest in the Labour group and Leonard’s video campaign was meant as a “thinly veiled attack” on Starmer.
A Labour source on the party’s left denied that is the intent, but said that people can read into it what they will.