Tens of thousands of Scottish National party members will cast their votes from Monday to select their party leader and Scotland’s next first minister, after a snap election forced by Nicola Sturgeon’s shock resignation. Polling began at noon on Monday with Humza Yousaf, Scotland’s health secretary, vying to become the country’s first minority ethnic first minister, against Kate Forbes, the finance secretary, and Ash Regan, a former community safety minister and the outlier candidate. Yousaf, 37, is regarded as Sturgeon’s preferred candidate after winning endorsements from the party’s most senior figures, including John Swinney, the outgoing deputy first minister; Stephen Flynn, the SNP’s Westminster leader; and roughly half of Sturgeon’s cabinet. The SNP’s first open leadership contest for nearly 20 years has been marked by an unprecedented level of infighting over the party’s track record and attacks on several of Sturgeon’s most cherished policies, particularly from Forbes and Regan. In the most significant development, Yousaf and Forbes both told SNP members it could take some years to build up sufficient support for independence to make it safe to stage a second independence referendum. Forbes, 32, who would be Scotland’s youngest first minister, launched a remarkable attack on Yousaf’s record as a minister in the first live TV debate, accusing him of significant failures while he was transport secretary, justice secretary and latterly health secretary. Forbes, an observant member of the socially conservative Free Church of Scotland, had already had a tempestuous start to her campaign after disclosing she did not support same-sex marriage, would not fight to overturn a UK government block on Sturgeon’s changes to gender recognition laws, and did not believe children should be born outside marriage. However, a number of polls of ordinary voters and SNP voters found Forbes is the most popular of the three candidates; smaller polling samples of SNP members, which were not demographically representative, found Yousaf and Forbes were essentially neck and neck. In a statement to mark the start of polling, Yousaf had a dig at Forbes. “Inclusivity, equality and respect for everyone” were central to the SNP’s recent successes, he said. “We cannot afford to have a leader who pulls us off that progressive path.” Forbes said opinion polling showed she commanded confidence across the electorate. “No one can accuse me of not delivering, of bending under pressure or being unprincipled,” she said. Over the weekend, Yousaf sought to attract hardline independence activists in the party who may be attracted by Regan’s pledge to repeatedly use elections as substitute referendums. He suggested he could call a snap Holyrood election to force the UK to negotiate, but only if popular support for independence was consistently high; SNP ministers say privately that would mean at least 60% support for a long period. The election is being carried out using the multi-choice single transferrable vote system, which means that if Yousaf fails to win the contest on first-choice votes, Forbes could win in the second round if Regan’s supporters make her their second choice. Uncertainties about how many SNP members may vote have grown after the Mail on Sunday reported at the weekend that only 78,000 people were eligible to vote, well below the 104,000 membership figure reported officially by the party last year. The voting lasts for two weeks, with the final result due to be announced on Monday 27 March. • This article was amended on 14 March 2023. An earlier subheading misspelled Humza Yousaf’s surname.