The months-long dance between President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE’s lawyers and Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE over a voluntary presidential interview has yet to bear fruit, raising the question of whether the special counsel will subpoena the president to testify as part of his Russia probe.

Those who know Mueller professionally say he would not hesitate to compel Trump to testify under oath before a grand jury if he considered it critical that his investigation include testimony from the commander in chief.

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Still, such a move would carry risks for both sides, agitating tensions between the White House and the special counsel’s office and potentially triggering a legal battle that could lead all the way to the Supreme Court.

Stephen Vladeck, a University of Texas law professor who has been closely watching the investigation, said that Mueller would subpoena Trump only if he “believes he has exhausted all other options and if he really thinks it is necessary.”

“Before you subpoena the president you want to make sure you have every single duck in a row,” Vladeck said.

Trump first signaled in January that he was eager to speak to Mueller under oath to show there was no collusion between his campaign and Moscow in 2016. Since then, the president’s lawyers say they have been trading proposals with Mueller’s team on the ground rules for an interview. No agreement has been reached.

Trump indicated in an August interview with Reuters that he is now leaning against sitting down with Mueller, fearing it could be a “perjury trap.”

If Trump declines a voluntary interview with the special counsel, Mueller could subpoena him — thereby compelling the president to answer questions under oath before a grand jury. Trump’s lawyers would most likely mount a legal challenge that could lead to a months-long court fight.

“It’s going to result in a lengthy legal battle, certainly, which is likely going to delay the resolution of his investigation,” said Randall Eliason, a former assistant U.S. attorney for D.C.

“If this means the investigation goes on for another year, delay is never a prosecution’s friend,” added Eliason. “You always want to get things resolved as expeditiously as possible.”

Issuing a subpoena also would likely lead to fresh attacks from the president and his Republican allies. Trump has regularly disparaged the Russia investigation as a political “witch hunt,” and on Thursday he told Bloomberg News that he believes the special counsel investigation is “illegal.”

“I think [Mueller’s prosecutors] would be crossing a line and, look, I also think that might play into the president’s hands,” said Ford O’Connell, a Republican strategist. “President Trump wants to fight this out in public because of what he, or the administration, sees as brutal prosecutorial tactics by the special counsel.”

But challenging a grand jury subpoena could also risk giving the appearance that Trump is not cooperating with the investigation, which his critics would undoubtedly seize on.

Legal analysts agree that it’s a step that Mueller, like any experienced prosecutor, is willing to take if Trump’s testimony is vital to ending the probe.

The question of whether a sitting president can be subpoenaed is subject to legal debate.

Past cases, specifically Nixon v. United States and Clinton v. Jones, suggest that the courts would rule in favor of the subpoena power. But neither of those cases dealt directly with subpoenaing a sitting president to testify before a grand jury.

Mueller is likely to want to question Trump on matters concerning the obstruction inquiry, such as the president’s firing of FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE, in order to establish whether Trump acted with “corrupt intent” to impede the investigation. Other questions would address Trump’s knowledge of the June 2016 Trump Tower meeting and speak to the broader collusion inquiry.

Either a voluntary interview or grand jury testimony could expose the president to legal peril, though in different ways. An interview is not typically conducted under oath, but subjects can be charged with making false statements if they lie to investigators.

Testifying under oath before a grand jury makes subjects vulnerable to perjury charges. In a grand jury appearance, subjects are also precluded from having their attorneys present.

Those risks are something that the president and his legal team are aware of and taking into consideration as they negotiate with the special counsel.

“I'm not going to be rushed into having him testify so that he gets trapped into perjury,” Rudy Giuliani, Trump’s lead attorney, told NBC News in August. “And when you tell me he should testify because he’s going to tell the truth, that he shouldn’t worry, well that’s so silly because it’s somebody’s version of the truth, not the truth."

If subpoenaed to testify under oath, a subject can invoke the Fifth Amendment to avoid self-incrimination and even refuse to testify, though doing so could carry political consequences for the president.

“The difficulty for Trump is, if he pleads the Fifth, he is now admitting he has criminal exposure,” said Seth Waxman, a former federal prosecutor in the U.S. attorney’s office in D.C.

Waxman said he expects Mueller to subpoena Trump if the president does not agree to a voluntary interview, though he noted that the special counsel would likely want to secure other key witnesses to cooperate in the probe before taking that momentous step.

“My suspicion is it is going to be after the new year at some point,” Waxman said.

The special counsel’s office declined to comment on the status of negotiations with Trump’s lawyers. Giuliani was not available for comment.

It is unclear when Mueller will complete his investigation, though the White House has encouraged its swift end. Mueller’s team faces its second court test in Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE’s criminal trial in Washington this month and has recently interviewed associates of former Trump adviser Roger Stone Roger Jason StoneThe agony of justice Our Constitution is under attack by Attorney General William Barr Justice IG investigating Stone sentencing: report MORE.

Mueller has already indicted two dozen Russians on hacking and fraud charges in the plot to interfere in the 2016 presidential election, but he has not addressed the questions of collusion or obstruction.

Some observers doubt that Mueller will need Trump’s testimony to complete his investigation, which is expected to culminate in a report to Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE, who is overseeing the probe.

Still, others suspect Mueller has not subpoenaed Trump because he may have become a target of the investigation; Mueller, equipped with a broad mandate to probe any matters that arise from his investigation, has reportedly been looking into the president’s finances.

Justice Department guidelines say that federal prosecutors generally should not subpoena targets of investigations, though they can if they give an individual written notice of being a target.

“I have believed since Day One that if Mueller thought subpoenaing the president was the right approach, he would have done it six months ago instead of this never-ending negotiation,” said Glenn Kirschner, a former federal prosecutor who worked with Mueller.

“He would have subpoenaed him if he wasn’t a target,” Kirschner said. “Mueller has no problem fighting this battle up to the Supreme Court.”