DAKAR - Senegal is heading for its most contentious election in recent history on Sunday overshadowed by political violence and a constitutional row that could sully its enviable reputation as West Africa’s most stable democracy. President Abdoulaye Wade is seeking a third term against a field of more than a dozen challengers and he appears to have the edge over a divided opposition. His candidacy has sparked deadly protests from opponents who say it flouts a two-term limit introduced by constitutional reform, and has also drawn criticism from trade and aid partners France.