In OTL, George Mifflan Dallas was the vice president under James K Polk. He had a long-standing rivalry with James Buchanan, who'd later become the 15th president. With this rivalry in mind, I'm curious as to how he could've become president instead of Buchanan, and what would have to happen for that to be the case. With a POD of the 1840 election, how could he be elected president(but not as an accidental president)? What kind of presidency might he make compared to his rival Buchanan?