[x]

Not much was left of the Whigs by 1858.

There may have been a rump who didn't go over to the Democrats or the American (Know Nothing) Party, and they would have been the ones to endorse PGTB, the Democratic candidate.

Beauregard was a lifelong Democrat according to one source.

[Pelham]

wiki has him running for office only the one time, for New Orleans mayor in 1858 with the support of both Whigs and Democrats. They cite this book in their footnote:

Williams, T. Harry. ‘P.G.T. Beauregard: Napoleon in Gray.’ Baton Rouge: Louisiana State University Press,