Cassius Clay was something of an embarrassment to Henry Clay in the 1844 presidential campaign. Cassius Clay "sought to persuade antislavery northerners to support his distant cousin instead of James G. Birney, the Liberty party candidate. In a letter published by the *New-York Daily Tribune* on August 13, Cassius Clay claimed that, while Henry Clay opposed immediate emancipation, 'his feelings are with the cause.' But Henry Clay promptly issued a strongly worded public denial. 'Mr. C. M. Clay’s letter was written without my knowledge, without any consultation with me, and without any authority from me,' he wrote. 'So far as he ventures to interpret my feelings, he has entirely misconceived them.' Citing his own previous statements, Clay argued further 'that Congress has no power or authority over the institution of Slavery;' and 'that the existence, maintenance and continuance of that Institution depend, exclusively, upon the power and authority of the respective States, within which it is situated.'"
http://www.oah.org/site/assets/documents/02_JAH_2003_kornblith.pdf
Probably the whole affair hurt Henry Clay with both pro- and anti-slavery voters, the former wondering if Cassius Clay might not be on to something, the latter incensed by the vehemence of Henry Clay's repudiation. I am not saying that without Cassius Clay around, Henry Clay would have won New York and therefore the election, but it is conceivable.