John McCain clinched the Republican presidential nomination Tuesday, an extraordinary comeback for a candidate whose White House hopes were dashed eight years ago and whose second bid was left for dead eight months ago. "The most important race begins," he said in an Associated Press interview.According to the AP count, the four-term Arizona senator surpassed the requisite 1,191 GOP delegates as voters in Ohio, Vermont, Rhode Island and Texas put him over the threshold. The triumph came one month after his Super Tuesday coast-to-coast victories gave him an insurmountable lead in the delegate hunt and forced his chief rival, Mitt Romney, to drop out of the race.