Kieran Murray and Alistair Bell MEXICO CITY (Reuters) - Mexico's presidential election is too close to call between a leftist anti-poverty campaigner and the conservative ruling party candidate locked in a tie, a respected exit poll said on Sunday. The extremely close vote raised fears of a political crisis if any of the main candidates challenge the results and call street protests. Pre-election polls had showed Felipe Calderon of the ruling party and Andres Manuel Lopez Obrador, the left-wing former mayor of Mexico City, in a virtual tie. Exit polls from Mexico's two main television station and the El Universal...