"Clay lost to Polk by only 30,000 votes, which suggests that there was limited support for annexation (in fact it was primarily rooted in the south). Yet President Tyler interpreted Polk's victory as a mandate for expansion, which prompted him to push through annexation as one of his final acts, with help from expansionists in Congress. If Clay had won the election, Tyler would not have annexed Texas, and the United States would not have entered into war with Mexico – at least not in 1845. Had Clay won, we might assume that the nation would have avoided annexation, and by extension war with Mexico." - Mark H

Texas may have become part of the US even without the Mexican-American war. - John (bird whisperer)