But Ms. Underwood’s office lacks the authority to prosecute federal matters. So, when she filed the lawsuit, she simultaneously sent letters to the Internal Revenue Service and the Federal Election Commission asking those agencies to investigate the alleged violations of federal tax laws and campaign finance laws, respectively.

And, for good measure, on the letter to the F.E.C., she copied two top officials from the Justice Department’s Public Integrity Section, which is charged with investigating and prosecuting criminal violations of election laws.

The copying of the letter to the Justice Department attracted wide notice in Washington’s close-knit election-law bar, as did the claim in the lawsuit that the use of the Trump Foundation to benefit the Trump campaign “was willful and knowing.”