Any indictment of the lawyer — Michael Sussmann, a former federal prosecutor and now a partner at the Perkins Coie law firm, and who represented the Democratic National Committee on issues related to Russia’s 2016 hacking of its servers — is likely to attract significant political attention.

Donald J. Trump and his supporters have long accused Democrats and Perkins Coie — whose political law group, a division separate from Mr. Sussmann’s, represented the party and the Hillary Clinton campaign — of seeking to stoke unfair suspicions about Mr. Trump’s purported ties to Russia.

The case against Mr. Sussmann centers on the question of who his client was when he conveyed certain suspicions about Mr. Trump and Russia to the F.B.I. in September 2016. Among other things, investigators have examined whether Mr. Sussmann was secretly working for the Clinton campaign — which he denies.