By March 1977, Labour had become a minority government after two by-election defeats cost them the three-seat majority they had won in October 1974, and from March 1977 to August 1978 Callaghan governed by an agreement with the Liberal Party through the Lib–Lab pact. Callaghan had considered calling an election in the autumn of 1978, but ultimately decided that imminent tax cuts, and a possible economic upturn in 1979, could favour his party at the polls and delayed the election until the following year. Although published opinion polls suggested that he might win, private polls commissioned by the Labour Party from MORI had suggested the two main parties had much the same level of support. However, events would soon overtake the Labour government and prove Callaghan's decision to delay an election to be a costly mistake. A series of industrial disputes in the winter of 1978–79, dubbed the "Winter of Discontent", led to widespread strikes across the country and seriously hurt Labour's standings in the polls while boosting support for the Conservative opposition. When the Scottish National Party (SNP) withdrew support for the Scotland Act 1978, a vote of no confidence was held and passed by one vote on 28 March 1979, forcing Callaghan to call a general election. As the previous election had been held in October 1974, Labour could have held on until the autumn of 1979 if it had not been for the lost confidence vote. Margaret Thatcher had won her party's 1975 leadership election over former leader Edward Heath, taking over the leadership of the party in February 1975, four months after the party's failure to win the October 1974 election. Heath had led the party for a decade but lost three of the four elections he contested. David Steel had replaced Jeremy Thorpe as leader of the Liberal Party in 1976, after allegations of homosexuality and conspiracy to murder his former lover forced Thorpe to resign. The Thorpe affair led to a fall in the Liberal vote, after what had been thought to be a breakthrough in the February 1974 election. Campaign This was the first election since 1959 to feature three new leaders for the main political parties. The three main parties all advocated cutting income tax. Labour and the Conservatives did not specify the exact thresholds of income tax they would implement but the Liberals did, claiming they would have income tax starting