Senate Judiciary Committee Chairman Chuck Grassley is putting a hold on top State Department appointments -- including the nominee for the department's fourth-highest post -- until he gets the answers he's seeking from a former top aide of Hillary Clinton tasked with helping determine which of the former secretary of state's emails should be made public.

The Iowa Republican -- who also is investigating the special employment status afforded to Clinton confidant Huma Abedin while at State -- has slammed the department for its "continued intransigence and lack of cooperation" throughout the inquiry, which dates back to June 2013. Critics, including Senate Democratic Leader Harry Reid, have derided the probe as a politically motivated bid to undermine the Democratic presidential candidate's campaign for the White House.

For its part, the State Department says it has responded to Grassley's questions "in 16 formal letters and many briefings, calls and emails," but remains overwhelmed by the volume of requests.

Grassley, who last week released holds on 20 career Foreign Service Officers, is now turning to bigger fish in a bid for leverage to get more cooperation from the department.

He is blocking the nomination of Thomas Shannon to replace Wendy Sherman as under secretary for political affairs, the No. 4 post in the department. In addition, his office told Fox News he has placed holds on the nominations of Brian James Egan for legal adviser and David Malcolm Robinson for assistant secretary for conflict and stabilization operations and coordinator for reconstruction and stabilization. 

A hold is a procedural tactic senators often employ to extract information or other concessions from the administration. President Obama himself made use of holds as a senator from Illinois, blocking nominations to the EPA during the Bush administration over objections to lead paint regulations.

Grassley’s holds came as he fired off a letter to former Clinton aide Heather Samuelson, posing 19 questions about the process used to screen the emails for the former secretary of state.

He also asked Samuelson what kind of security clearance she had at the time, given that hundreds of Clinton's emails have been shown to contain classified information.

"Given the importance of securing and protecting classified information ... it is imperative to confirm when, how, and why you, and any of your associates, received a security clearance in connection with your work on behalf of Secretary Clinton and whether it was active while you had custody of Secretary Clinton’s emails," Grassley wrote in the letter, first reported by Politico.

"Further, it is imperative to understand your background in determining what is and what is not a federal record, since you apparently played a major role in assisting Secretary Clinton in making a decision as to which emails to delete."

Clinton has come under heavy fire for routing official emails through a personal server during her time as secretary of state. The Democratic front-runner's aides have also faced scrutiny for their roles in determining which messages to turn back over to the agency, which has been slowly making them public under a court order.

Critics have accused Clinton of putting sensitive government information at risk under the arrangement. Separately, the FBI has been investigating whether the setup resulted in the mishandling of classified information.

The State Department insisted it is trying to work with Grassley’s office.

"Over the course of the last several months, the mounting requests from the Chairman of the Senate Judiciary Committee have contained nearly 200 detailed questions and 65 unique document requests," department spokesman Alec Gerlach said in a statement. "The department is committed to working with the committee and providing responses as quickly as possible, but the growing effort needed to accommodate these requests is overwhelming the resources we have available."

A Grassley spokeswoman described the level of cooperation as “sparse.”

Senate Democrats have urged Grassley to drop his objections.

"The senior Senator from Iowa comes to the floor and talks about the proper use of taxpayer resources," Reid, the Democratic leader, said earlier this month. "He should walk into his bathroom and look into the mirror and find out what he’s doing about the proper use of taxpayer resources. He should be willing to tell us about the resources his committee is spending to investigate Secretary Clinton."

Fox News’ Mike Emanuel contributed to this report.