Did Millard Fillmore Ever Endorse the Wilmot Proviso?

Here's a little mystery.

Elbert Smith asserts several times in his The Presidencies of Zachary Taylor & Millard Fillmore that, before he was nominated for the vice presidency in 1848, Fillmore expressed support for the Wilmot Proviso. For example, on page 165, Smith asserts that "Fillmore had once been a vocal supporter of the Wilmot Proviso, but listening to Clay and Webster eloquently denounce it [during the senate debates leading to the Compromise of 1850] . . . had undoubtedly helped influence him to consider it as unnecessary." On page 23, Smith states that "Conscience Whigs [at the 1848 Whig convention] . . . knew that Fillmore had consistently supported the Wilmot Proviso."

Are these statements true? Michael Holt seems to disagree.

In his The Rise and Fall of the American Whig Party (pp. 357-58), Holt describes efforts of Democrats to locate "incriminating evidence" on Fillmore. They found some. Among other things, Fillmore "had consistently voted against the gag rule." In addition, "they found and printed an 1838 Fillmore letter to abolitionists endorsing congressional abolition of both slavery in the District of Columbia and the interstate slave trade."

What southern Democrats did not find, apparently, was any evidence that Fillmore had ever endorsed the Wilmot Proviso. "Southern Whigs . . . bombarded Fillmore with questions after the convention about whether he had ever publicly endorsed the Proviso, and they expressed enormous relief when he replied he had not."

Conceivably, with a lot of stretching, these statement are reconcilable. Perhaps Fillmore privately endorsed the Proviso to friends, but left no public paper trail. Alternatively, perhaps Smith or his source took Fillmore's letter endorsing abolition of the interstate slave trade as roughly equivalent to endorsement of the Proviso. Or perhaps Holt simply fails to tell us that Fillmore's denial was a lie (although I find that hard to believe).

Can anyone out there solve the mystery?