Clay was opposed to the annexation of Texas because he knew it would lead to a war with Mexico. He knew that the lands taken from Mexico in war would reopen the Missouri Compromise for renegotiation. He remembered just how close the nation came to disunion in 1820 over that negotiation, and he believed that opening it up again could possibly lead to disunion and civil war. He clearly saw the risk of annexing Texas.
Polk ran on one issue only: He would annex Texas. If he could do it in his first term, he would not run for re-election.
Remini believes that if the election had not been stolen, Clay would have left the Texas issue alone, there would have been no war with Mexico, and the Civil War could have been avoided or at least delayed.
He also believed that had Clay managed to hold off his tuberculosis and live another decade, he could have played "good cop" in the Senate while Lincoln played "bad cop" in the White House, thus managing to prevent the South from seceding.
Had Clay carried NY (which Polk carried by only 1.05%), he wouldn’t have needed LA (which Polk carried by 2.60%, so, if he stole it, he must have cheated like crazy). Clay lost NY because James Birney of the anti-slavery Liberty Party got 3.25% in NY. Had Clay spoken out more strongly against the expansion of slavery, he might have won (although since Clay only carried TN by 0.10%, he probably would have needed to carry IN or PA as well to make up fir the loss of TN had he spoken out that way).
Imagine an America without the Texas annexation and the Mexican War. Would we ever have acquired the west? I suppose Mexico may have eventually offered to sell it to us.
I read somewhere that the Gadsden purchase could have been MUCH larger, including Baja and several Northern Mexican states, but one side or the other eventually balked.