The National Archives indicated that the timeline was not realistic.
The documents at issue, sought by Trump's fellow Republicans, relate to Kavanaugh's service from 2001 to 2003 as a White House lawyer under Republican former President George W. Bush.
The documents could be produced earlier via a separate source: the Bush presidential library, which is conducting its own review.
"We estimate that we can complete our review of the textual records and the subset of White House Counsel Office emails "from" Kavanaugh (approximately 49,000 emails)-totaling roughly 300,000 pages - by approximately August 20, 2018, and now expect to be able to complete the remaining 600,000 pages by the end of October 2018", National Archives general counsel Gary Stern wrote in the letter addressed to Grassley.
"As Chairman Grassley said this morning, he intends to hold a hearing sometime in September", said the aide.
Republicans have protested that the number of documents is more than the number requested for the previous five Supreme Court nominees combined. "That's because Senate Republicans plan to rely on the documents provided by the Bush legal team and not wait for the National Archives to complete its review process".
Republican Rep. Kevin Cramer, who's challenging Heitkamp this fall, doesn't have a formal say in Kavanaugh's confirmation but voiced his support for the judge shortly after Trump announced the pick last month.
Dozens of protesters filtered into the Senate office building on Wednesday and lined the hallways outside senators' offices in an attempt to stop Republican senators from meeting with Kavanaugh.
"There's no chance in hell Mitch McConnell holds this vote after the election".
Democrats say the documents they are requesting, including the staff secretary papers, are crucial to understanding all aspects Kavanaugh's background, particular his role in forming policy under Bush.
But Republicans have requested only a portion of the records, leading Senate Minority Leader Chuck Schumer to accuse Republicans of having "cast aside Democratic wishes for openness and transparency". He said he could not reach an agreement with Feinstein over the scope of the documents request. "The Bush Library will produce to us over 125,000 pages today", said the aide. Orrin Hatch (R-Utah) said at a Capitol Hill press conference on Thursday. "To be clear, President Bush has offered this as a courtesy to the Committee to assist in a timely assessment of Judge Kavanaugh's nomination".
Standing in front of rows of empty boxes labeled "Kavanaugh files", Hatch and other GOP members of the Senate Judiciary Committee argued that their request to the National Archives would give the Senate more than enough material to adequately evaluate the judge's record in what Grassley called a "fair and thorough process".