Is this sentence correct? "Richard Nixon—whom Mazo claimed had lost the election because of the fraud—asked for the series to be terminated."

Is the following sentence grammatically correct?

Richard Nixon—whom Mazo claimed had lost the election because of the fraud—asked for the series to be terminated.

I'm wondering whether it should be corrected to:

Richard Nixon—who Mazo claimed had lost the election because of the fraud—asked for the series to be terminated.

Who—the subject pronoun—is correct.

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In order to determine whether the relative pronoun functions as the subject or the object of a clause, we can apply a little test. Let's start with a simpler example:

Richard Nixon—who/m Mazo supported—asked for the series to be terminated.

who Mazo supported --> *Mazo supported he (incorrect; don't use subject pronoun)

whom Mazo supported --> Mazo supported him (correct; use object pronoun)

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Your example is more complex, but let's apply the same test:

Richard Nixon—who/m Mazo claimed had lost the election—asked for the series to be terminated.

whom Mazo claimed had lost the election --> *Mazo claimed had lost the election him (incorrect; don't use object pronoun)

who Mazo claimed had lost the election --> *Mazo claimed had lost the election he (incorrect; don't use subject pronoun)

Well, wait a minute, if neither of those works then . . . we have a clue. The pronoun is the subject of its own clause within the clause:

Richard Nixon—who/m Mazo claimed had lost the election—asked for the series to be terminated.

whom Mazo claimed had lost the election --> Mazo claimed [that] him had lost the election (incorrect; don't use object pronoun)

who Mazo claimed had lost the election --> Mazo claimed [that] he had lost the election (correct; use subject pronoun)