The Scottish National party will find it far harder to govern unless the next first minister agrees to work with centre-left parties, a co-leader of the Scottish Greens has said. The Greens were in a power-sharing deal with the SNP until Humza Yousaf unilaterally ended it last week, precipitating his downfall on Monday. In an interview with the Guardian, Patrick Harvie said Yousaf’s successor as first minister would face “knife-edge vote after knife-edge vote” or be forced to do deals with the anti-independence Conservatives unless they kept the Scottish Greens on side. He refused to speculate on whether the Greens favoured John Swinney or Kate Forbes of the SNP leadership frontrunners, but his remarks will be widely seen as a warning to Forbes and her supporters. Forbes is a social conservative opposed to many policies championed by the Greens around gender identity, abortion clinic buffer zones and climate action. The Greens have repeatedly said they are very unlikely to support her as first minister. “Whoever the SNP choose is still going to have to command a majority in parliament, they are still going to have to reach out,” he said. “There is a clear consensus across Scottish politics for a broadly left-of-centre economic agenda which has fairer taxation, redistributive policies like the Scottish child payment, and does what it can to stand up to austerity.” Harvie and his colleague Lorna Slater, the Scottish Greens’ other co-leader, were ousted as ministers last week after Yousaf tore up the Greens-SNP coalition deal, known as the Bute House agreement. Yousaf announced he was quitting on Monday after the Greens said they would no longer work with him at Holyrood, leaving him dependent on Alex Salmond’s fringe nationalist party, Alba, which has just one MSP in Holyrood, the former SNP minister Ash Regan. Forbes, a former finance secretary, confirmed on Tuesday she was considering standing but it is widely thought she will withdraw if Swinney, a former SNP leader and the clear favourite to win, offers her a significant job in his cabinet. In his first newspaper interview since being sacked, Harvie said the Bute House agreement had been under strain since Yousaf unilaterally decided to freeze council tax rates in Scotland last October, in breach of the deal. That damaged trust, Harvie said. Then this month the Scottish government scrapped its target to cut Scotland’s carbon emissions by 75% by 2030. On the same day, the NHS announced that the Sandyford clinic, Scotland’s only gender service for young people, was suspending the use of puberty blockers after publication of the Cass review’s findings for NHS England. “This was more than a bump in the road. This was a very serious jolt,” Harvie said. Those decisions caused Scottish Green party members to call for a vote on the Bute House agreement, and Harvie said he would resign as co-leader if they voted to abandon it. Harvie said that in parallel Yousaf was coming under intense pressure from a centre-right grouping inside the SNP to abandon the Bute House agreement because they were ideologically opposed to many of its measures. These included the move initiated by Nicola Sturgeon, Yousaf’s predecessor, to call a climate emergency and accept that North Sea oil and gas drilling had to be cut. “I think there was a sense that very clearly some people in the SNP wanted to go back to the old tunes. The council tax freeze was going back to one of the old tunes, and I think for some of them supporting the oil and gas industry felt the same,” Harvie said. Although the SNP’s critics of the Bute House agreement “make a great deal of the fact they represent rural areas”, their opposition was ideological, Harvie said. “Actually, there are some very progressive people in the SNP who represent rural areas,” he said. “Many of the same people on the right are every bit as hostile to things like, you know, the heat and buildings agenda, progressive taxation, whether it’s social justice, in terms of LGBTQ equality. There’s a general sense of small-c conservatism throwing its weight around.” He said the SNP was facing an existential problem and lost its “mojo” because the “single organising mission” of a quick second independence referendum had faded so far that different political factions were emerging. Harvie said he worried that the backlash within the SNP, partly fuelled by fears that the Conservatives will grab seats in north-east Scotland by championing the oil industry’s interests, meant the climate and nature emergencies were slipping down the political agenda. “There is a tendency across many political parties to downgrade those issues,” he said. “That’s happening at UK level with both the major parties, it’s happening a number of other European countries.” There was a similar danger that the “anti-woke mind virus” and climate conspiracies could take root in Scotland, by amplifying criticisms of progressive policies around abortion clinic buffer zones, gender recognition and on climate policy, he said. Now that the Greens were out of government, Harvie said, the party needed some “introspection” on its priorities. One key issue would be mobilising the wider environment movement to campaign much harder for the Scottish government and Holyrood to support pro-climate, green policies, he said.