Sincere... Clinton sincere... depends on what the meaning of sincere is.
The releveant OED definition of "sincere" is "Characterized by the absence of all dissimulation or pretence; honest, straightforward." Referring to persons, the word in English goes back to 1539, and the (unlinkable) OED has a lot of historican examples, but one stands out:
1824 Byron Don Juan: Canto XVI xcvii. 112 For surely they're sincerest, Who are strongly acted on by what is nearest.In context:
So well she acted all and every partRight now, the closets are most near/To clean them must be quite sincere.
By turns — with that vivacious versatility,
Which many people take for want of heart.
They err — 't is merely what is call'd mobility,
A thing of temperament and not of art,
Though seeming so, from its supposed facility;
And false — though true; for surely they're sincerest
Who are strongly acted on by what is nearest.