I once wrote a paper on Henry Clay, and for a while was obsessed with him. But while I despise Jackson, I came to strongly dislike Clay for his lack of courage in confronting slavery. This, more than anything else, kept him out of the presidency.
Polk was pro-slavery, more so than Clay, so I would think that abolitionists would support Clay over Polk. However, enough voted for Birney to keep Clay from winning.
I don’t think any abolitionist president was elected between JQ Adams in 1824 and Lincoln in 1860 (with the possible exception of Van Buren in 1836). It wasn’t exactly a time of principled national politicians.