The mailers urge voters to back a Libertarian or Independent voter over Mr. Hawley and do not include required disclaimers identifying who paid for them.
“Voters telling me about fake & illegal mailers pro-Dem group is putting out. There should be an FBI investigation,” Mr. Hawley, the state attorney general, wrote on Twitter.
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He called for Democrat incumbent Sen. Claire McCaskill to cooperate in a probe.
“As the beneficiary of this Kavanaugh-style illegal smear campaign, I hope @clairecmc will cooperate with the investigation fully #MoSen,” tweeted Mr. Hawley.
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