As Canadian political and business leaders expressed optimism the proposed Keystone XL pipeline will win approval under a re-elected President Barack Obama, environmental opponents and the U.S. ambassador to Canada cautioned the energy megaproject isn’t a slam dunk. Obama’s Republican challenger, Mitt Romney, had vowed quick approval of the $7.6-billion pipeline if he had won Tuesday’s presidential election. But Obama, who earlier this year rejected TransCanada Corp.’s initial application because it needed more environmental review, has remained noncommital about the fate of the line, which would ship Alberta oilsands product to the U.S. Gulf Coast. In Ottawa, federal Natural Resources...