A Democratic committee aide said he assumed that the new emails from Blumenthal were not provided by the State Department because the committee's request to the department was more narrowly cast.
The deposition and the intense press interest it will attract provide a reminder that past controversies surrounding Clinton will loom over her campaign. "If Barack Obama was no-drama-Obama, Bill and Hillary Clinton are all drama all the time," political analyst Tobe Berkovitz said.
He said it's a reminder of the 90s-vintage Clintons, even if Blumenthal himself is hardly a household name.
"To most voters, Sidney Blumenthal is a name from the past that they have pretty well forgotten, if they even knew it to begin with. He was part of the inside baseball, inside the Beltway players inside the [Bill] Clinton administration," said Berkovitz, a mass communications and public relations professor at Boston University.
But he said it "starts dredging up all the Clinton baggage, all the drama, all the characters."
While the deposition will be behind closed doors, it's fueling the very public collision between Republicans, who say important questions remain unanswered, and Democrats, who say the probe is a nakedly political attack against Hillary Clinton.
In a lengthy statement Monday evening, the pro-Clinton super PAC Correct The Record accused Republicans of "trolling for anything they can use to harm Hillary Clinton no matter how off-topic or outlandish."
"In this case, they have subpoenaed a cooperative witness for show—a witness who has not one thing to do with what happened in Benghazi or the government's reaction to it—because hurting Hillary Clinton's political prospects is now the all-consuming passion of House Republicans, not getting to the bottom of what happened in Benghazi—as if that was ever the real purpose of this committee to begin with," said Brad Woodhouse, a veteran Democratic operative who is the group's president.
Emails turned over to the panel early this year and made public in May show detailed messages about Libyan intelligence, military, and political issues that Blumenthal sent to Clinton, who passed a number of them along to top aides.
They're included in 850 pages of Clinton's Libya-related emails that the State Department provided the Select Committee in February and released publicly last month.
Blumenthal was employed by the Clinton Foundation and as a consultant to pro-Clinton advocacy and political groups, and he also advised companies seeking contracts with Libya's post-Gaddafi transitional government, The New York Times reported in May.
"We know Mr. Blumenthal corresponded with the former Secretary regarding matters directly related to the scope of this investigation. But we have many unanswered questions when it comes to how the information he provided was compiled and the overall role he was playing," said Rep. Susan Brooks, a GOP member of the panel who served as U.S. Attorney for the Southern District of Indiana.