Well, the Free Soilers might support him as a lesser evil than Pierce, but the problem is that this will hurt him in the South, and will be a mixed blessing even in the North (where even in OTL some conservative pro-Fillmore Whigs evidently refused to vote for Scott in New York and other states).
His attempt to woo the Catholic vote was extremely clumsy, and cost him some Protestant support without winning him any substantial numbers of Catholics. But avoiding this mistake is hardly enough to defeat Pierce.