People with knowledge of the investigation said it could last at least another year — pointing to ongoing cooperation from witnesses such as former Trump campaign adviser George Papadopoulos and former national security adviser Michael Flynn, as well as a possible trial of two former Trump campaign officials. The special counsel's office has continued to request new documents related to the campaign, and members of Mueller's team have told others they expect to be working through much of 2018, at a minimum.
The dynamic threatens to intensify the already inflamed political atmosphere enveloping the investigation into Russia's meddling in the 2016 election. Even as White House lawyers have pledged to cooperate with Mueller, Trump and his allies have accused the Justice Department and FBI of bias and overreach.
The latest salvo came this past weekend, when a lawyer for the presidential transition accused Mueller of wrongfully obtaining thousands of emails sent and received by Trump officials before the start of his administration. The special counsel's office said all the material was legally obtained.
The meeting's outcome could deepen tensions as many Trump supporters question Mueller's credibility and Democrats express fear that the president will seek to fire the special counsel.
Ty Cobb, the White House lawyer overseeing the response to the Russia investigation, did not respond to phone calls and text messages seeking comment. Peter Carr, a spokesman for the special counsel, declined to comment.
White House lawyers have told the president he could be exonerated as early as the beginning of the year, after previously reassuring him that he would be cleared by Thanksgiving and Christmas, as The Washington Post previously reported. They have stated publicly that all White House interviews are over and that Mueller's team is no longer seeking White House documents.
In the meeting this week, they plan to ask Mueller's investigators if they need more information before reaching a conclusion that the probe as related to Trump is complete, according to a person familiar with the Trump team's plan who spoke on the condition of anonymity to describe private conversations.
The question that White House lawyers will pose to the special counsel's office, according to the person: "You've had all these witnesses, all these records. Is there anything else you need from the White House?"
Until now, Trump's legal team has repeatedly pledged cooperation with Mueller's office. A White House spokesman said there was no plan to change the strategy.
Trump's legal team has reassured him that it sees no evidence of collusion or obstruction in the records that it turned over, White House advisers said. While the lawyers have told Trump that they expect the investigative team will continue its work related to former campaign chairman Paul Manafort deep into next year, as well as possibly Flynn, they said they believe Mueller should be close to wrapping up the focus on the current White House.
Trump himself has expressed frustration with the probe but has shown optimism that it will not touch him. He told associates recently that he harbors no deep concern over the investigation and noted that his lawyers talk with Mueller's team regularly, according to a person who spoke with Trump last week and spoke on the condition of anonymity to describe a private conversation.
When pressed by two advisers to take the matter more seriously and asked why he is so confident in his lawyers, Trump brushed off the concerns. "He is living in his own world," the person said, predicting that Trump would erupt at some point in 2018 if the probe continued to drag on.
Another associate said that even in private, Trump is "confident, even arrogant" that he has done nothing wrong.
"There is no collusion," the president said at the White House on Sunday, after saying he was not planning to fire Mueller.
Among people familiar with the probe, there is widespread skepticism that the special counsel's investigation is drawing to a close.
Already, Manafort and his former deputy Rick Gates have been indicted on money-laundering and other charges related to work they did in Ukraine before the 2016 race. Two others have pleaded guilty and are cooperating, giving investigators new leads to pursue. Agents have gathered huge volumes of documents and conducted their first round of interviews with White House officials.
As recently as last week, Mueller's team was still asking questions about the firing of James B. Comey as FBI director, one person said.
Legal experts said Mueller would have little incentive to clear the president or other White House aides while he is seeking more information from witnesses.
"I think it's possible Mueller's team could give them an idea of how much longer they anticipate their investigation will last," said Peter Zeidenberg, the former deputy special counsel who helped investigate the leak of Valerie Plame's covert role as a CIA operative. "I would be shocked if they have a timeline anything similar to what we've heard coming from the White House."
"As far as a clean bill of health, I can't imagine they are going to be prepared to make a decision like that at this point," he said of the special counsel's team. "They are not going to be in a position to make that call until they finish this case and finish discussing all the evidence they have."
Mark Corallo, a former spokesman for Trump's legal team, said he thinks it is unlikely that the probe wraps up by the end of the year, but he said he believes it could conclude in the spring. He said that Mueller is aware of the political implications surrounding his investigation.
"Bob understands you can't have a president who is living under this cloud of uncertainty," Corallo said, adding that he believes it is possible that the special counsel will at some point call Trump's lawyers and say, "We are done with the president. There is nothing there."
The high-stakes meeting between White House lawyers and Mueller's team comes as conservative lawmakers and pundits have intensified their demands for a second special counsel to investigate the FBI, pointing to text messages between two former FBI officials discussing their dislike of Trump.
"We are now beginning to understand the magnitude of the insider bias on Mueller's team," House Judiciary Committee Chairman Bob Goodlatte (R-Va.) said last week. Another member of the committee, Rep. Steve Chabot (R-Ohio), said Mueller and his team should wear "Democratic Donkeys or Hillary T-shirts."
A White House adviser said the president has enjoyed the attacks. In recent weeks, he has spoken to a number of Fox News hosts, Republican lawmakers and others who have castigated Mueller's team, the adviser said.
The attacks on Mueller's investigation grew this weekend after an attorney for the presidential transition told congressional investigators Saturday that thousands of pages of the organization's communications were provided to Mueller by the federal General Services Administration.
Trump's lawyers learned Mueller had the emails this month when witnesses were quizzed on the material. Some of the documents contained sensitive information that wasn't related to the Russia investigation, according to a person familiar with the material.
A GSA spokeswoman declined to comment. Mueller's team said it obtained all documents legally.
Tom Hamburger and Rosalind S. Helderman contributed to this report.