Which preposition to use with assertors

of Occurrences 26%

But the alternative is to admit that these Bacteria arise from germs in the air; and if they are thus propagated, the burden of proof that other like forms are generated in a different manner, must rest with the assertor of that proposition.

in Occurrences 1%

For, my lords, to find friends in adversity, and assertors in distress, is only the prerogative of innocence and virtue.

Which preposition to use with  assertors