Mr. Manafort joined the Trump campaign in March 2016, was promoted to campaign chairman and chief strategist that May and was ousted on Aug. 19, 2016 after heightened scrutiny over his ties to Russian-allied leaders in Ukraine. According to the indictment, Mr. Manafort and his protégé, Rick Gates, served as unregistered agents of Ukraine from at least 2006 to 2015. They laundered payments through American and foreign entities “from approximately 2006 through at least 2016.” They also made false and misleading statements to the Justice Department between Nov. 23, 2016 and Feb. 10, 2017.

Ms. Sanders claimed that the indictments have “nothing to do with the president’s campaign or campaign activity.”

In addition to the indictments against Mr. Manafort and Mr. Gates, George Papadopoulos, an adviser to the Trump campaign, pleaded guilty to lying to F.B.I. investigators about a conversation he had with a professor promising “dirt” that Russia had on Mrs. Clinton. Ms. Sanders downplayed Mr. Papadopoulos’ role in the campaign as an “extremely limited” volunteer position and suggested he acted independently and not in an official capacity.

Mr. Papadopoulos was named as a foreign policy adviser to the Trump campaign in March 2016 (he previously advised Ben Carson’s campaign) and was described by Mr. Trump in an interview with The Washington Post as an “excellent guy.”

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.”