Documents released by investigators show that an unnamed professor — later identified by a Senate aide as Joseph Mifsud — with ties to Russian officials told Mr. Papadopoulos in April 2016 that Moscow had “thousands of emails” belonging to Mrs. Clinton. That May, Mr. Papadopoulos told an unnamed “high ranking campaign official” in an email that “Russia has been eager to meet Mr. Trump for quite some time and has been reaching out to me to discuss.”

The official forwarded Mr. Papadopoulos’ email to another unnamed campaign official and said, “Let[‘s] discuss. We need something to communicate that DT is not doing these trips. It should be someone low level in the campaign as to not send any signal.”

Ms. Sanders claimed there is “clear evidence of the Clinton campaign colluding with Russian intelligence.”

Ms. Sanders is referring to revelations last week that the Clinton campaign and Democratic National Convention partially funded the so-called Steele dossier. That series of memos detailed unverified accounts of the Trump campaign coordinating with Russian officials during the election, as well as more salacious encounters between Mr. Trump and Russian prostitutes.

Perkins Coie, the law firm representing the Clinton campaign and the D.N.C., hired the research firm Fusion GPS to obtain research on Mr. Trump in the summer of 2016. Fusion GPS then hired Christopher Steele, a former British intelligence agent, who used Russian sources to compile his dossier.

As The New York Times’ Ken Vogel explained last week, there is nothing illegal about the Clinton campaign hiring a company to conduct research on its opponent, but its failure to fully disclose their ties to the research firm has raised ethical objections. Moreover, Fusion GPS was first hired by The Washington Free Beacon, a conservative website, in 2015 to conduct research on several Republicans, including Mr. Trump.

With no evidence, Mr. Trump said the “Obama campaign” paid Fusion GPS.

As news broke of the indictments, Mr. Trump sought to link Mr. Obama with Fusion GPS and the dossier.