Because expansion led to the inevitable question: would slavery be permitted in any new territories gained from expansion? The key issue was Texas

Because expansion led to the inevitable question: would slavery be permitted in any new territories gained from expansion?

The key issue was Texas. Texas had been an independent country since gaining its independence from Mexico in 1836. Should Texas be allowed to enter the Union? Since Texas already allowed slavery, it was presumed Texas would be admitted as a slave state.

Martin VanBuren was seen as the frontrunner for the Democratic Party. Many felt he'd get the party's nomination. He came out and said that he was opposed to admitting Texas.

Henry Clay seemed sure to win the Whig nomination. Not wanting to be outdone by VanBuren, he also came out as opposed to Texas statehood.

But VanBuren did not get the nomination. Instead, the first real "dark horse" candidate, James K. Polk, did. Polk came out in favor of admitting Texas.

When Clay heard this, he switched and said he favored Texas statehood as well. Clay may have lost votes because he was seen as wishy washy--and willing to do or say anything to get elected.