Socialists and Communists agreed yesterday to form their first alliance since the Spanish civil war in a bid to oust Prime Minister Jose Maria Aznar in a March 12 election. Negotiators hammered out a joint political programme under which the Communist-led United Left (IU) pledged to support its longtime rival, the main opposition Socialists, in trying to form a government after the general election. The IU also agreed to join forces with the Socialists in key races for the Senate, the upper chamber of parliament, in an effort to take away seats from Aznar's centre-right Popular Party. But a final round of marathon talks between the two parties failed to fulfil Socialist hopes for a more closely knit coalition to seek control of Congress, the lower house which wields the greater clout. The agreement in principle, inspired by a French alliance that swept Socialist Lionel Jospin to power in 1997, raises the stakes for Aznar, who now faces a threat of a unified challenge from the left. Just hours after negotiators finalised the pact, the IU's ruling committee voted overwhelmingly to accept it. Socialist leaders were due to meet on Friday to give their formal endorsement, launching the first alliance of its kind since the 1936-39 civil war, when a leftist republican government was crushed by General Francisco Franco.