Senegalese President Abdoulaye Wade’s main rivals pledged on Saturday to campaign together in an effort to force him to withdraw from an election next month.The group did not give details on how they would try to prevent Wade from running for a third term in the February 26 vote. But the pledge, a day before campaigning officially opened, underscored rising tensions in a country that has long been an island of stability in West Africa.Wade’s candidacy was approved last week by the country’s top legal body, but it has sparked riots on the streets and drawn criticism from foreign donors, especially the United States, which said the bid posed a risk to Senegal’s stability.