President Trump speaking about tax reform during a visit to Loren Cook Company in Springfield, Missouri. Thomson Reuters President Donald Trump's pardon of Joe Arpaio, the former Arizona sheriff, has raised questions about whether he will use a similar tactic in the Russia investigation.

The move has prompted concern that the president could nullify the FBI's special counsel's central enforcement mechanism in the Russia probe.

The FBI is scrutinizing many top Trump associates in its investigation into Russian interference in the 2016 presidential election.

President Donald Trump's decision last week to pardon Joe Arpaio, the former Arizona sheriff, has raised questions about the limits of presidential pardon power and whether he would try to use it to hinder the FBI's investigation into Russia's election interference.

A federal judge last year ordered Arpaio to stop targeting Latinos who had not committed a crime in his immigration-enforcement operations. Arpaio continued the practice — which was the subject of a racial-profiling case that spanned a decade — and was found guilty of criminal contempt of court late last month.

However, Trump pardoned the Arizona sheriff before he was even sentenced. The move has prompted concern that the president has effectively nullified the central enforcement mechanism of Robert Mueller, the special counsel leading the FBI's Russia probe: holding witnesses in criminal contempt of court if they defy a subpoena.

The FBI is now scrutinizing many of Trump's associates as it examines whether the Trump campaign colluded with Moscow during the 2016 election. Those include the president's longtime lawyer, Michael Cohen; his campaign chairman, Paul Manafort; his former national security adviser, Michael Flynn; and his son-in-law, Jared Kushner.

Trump could pardon them, but that would "extinguish" their ability to invoke their Fifth Amendment right to protect against self-incrimination based on their federal criminal liability, said Renato Mariotti, a longtime federal prosecutor. And Mueller could still compel them to testify with a subpoena.

If they refused to testify even after being subpoenaed, however, they would be held in criminal contempt. And "it is possible that Trump could pardon that contempt conviction," Mariotti said.

"You could then have an infinite cycle where Mueller subpoenas the person, they refuse and are held in contempt, and then Trump pardons," he added.

Trump at a meeting with manufacturing CEOs at the White House in February. Reuters

Mariotti cautioned, however, that while that possibility "may be a fun intellectual exercise to think about," he didn't "see it happening as a practical matter."

"The political downside would be too great," he said. "A more likely outcome would be that the associate would either take the Fifth asserting state criminal liability, or they would testify but would do whatever possible not to help Mueller."

Louis Seidman, a constitutional-law scholar who's a professor at Georgetown Law Center, said that while "there is a risk that Trump will also use his power to pardon his kids or others from contempt citations following their refusal to testify or produce documents," doing so would be even more legally problematic than pardoning Arpaio.

Seidman pointed to a 1925 Supreme Court case in which a unanimous court upheld a presidential pardon for criminal contempt. But the chief justice at the time, William Taft, specifically distinguished civil contempt and made clear "that civil contempt is not subject to the pardon power," Seidman said.

Legal scholars disagree about whether a Trump associate would be held in civil or criminal contempt for failing to comply with a subpoena in this case.

Mariotti said he thought that "someone in that situation is more likely to be subject to criminal contempt" because Mueller's investigation isn't a lawsuit but a grand-jury proceeding.

Seidman, however, characterized a failure to comply with a court order in this situation as "civil, indirect contempt," adding that there is a strong separation-of-powers argument against the president using pardons to excuse civil-contempt violations.

"If Trump tried to upset a civil-contempt citation against someone who failed to respond to a subpoena in the Russia probe, I think it would be pretty clear that he would be exceeding his powers," he said.

State criminal prosecution 'may turn out to be the best way'

Anyone pardoned by Trump — who can pardon only people convicted of federal crimes — could still be vulnerable to prosecution at the state level.

"The president's pardon is not unlimited — in fact, it is constrained by a couple of important factors," said Jens David Ohlin, a vice dean and law professor at Cornell Law School.

"State and local convictions can only be pardoned by a governor, not the president," he said. "So the possibility of a state criminal prosecution may turn out to be the best way of constraining this administration."

Ari Melber, an MSNBC host who was until recently the network's chief legal correspondent, raised that possibility on Tuesday, arguing that Trump's presidential pardons would not necessarily preclude Russia-related prosecutions.

"While presidential pardons can halt the federal case, local prosecutors could then pursue any Americans suspected of aiding Russia's election meddling," Melber wrote. "In fact, legal experts say presidential pardons could make that prospect more likely."

Trump and his eldest son, Donald Trump Jr. REUTERS/Lucas Jackson

US intelligence agencies that examined the hack of Democratic National Committee email servers last summer concluded it was a criminal intrusion orchestrated by the Kremlin. And Samuel Liles, the acting director of the cyber division of the Department of Homeland Security's Office of Intelligence and Analysis, testified in June that as many as 21 states' election systems were targeted during the 2016 election by hackers believed to be Russian.

Experts say a prosecutor could circumvent a Trump pardon if they found that one of the election crimes occurred within a particular state's jurisdiction.

They could argue, for example, that Donald Trump Jr.'s meeting with two Russian lobbyists at Trump Tower in New York in June 2016 — also attended by Manafort and Kushner — was "in furtherance of an underlying crime," Jennifer Rodgers, a former federal prosecutor in New York, told NBC.

Federal investigators are also examining whether Manafort evaded taxes or laundered any money he received from his decade-long work for Ukraine's pro-Russia Party of Regions.

Even if Trump eventually pardoned Manafort, states in which financial laws were broken could pursue penalties. Mueller has reportedly teamed up with New York Attorney General Eric Schneiderman to examine Manafort's business dealings and financial history.

Mariotti said that while he agreed with the proposition that states could pursue criminal cases against Trump's associates, "there is some 'irrational exuberance' where analysts make assertions about the scope of crimes that they have seen little evidence of."

Trump's own fate in the Russia probe, meanwhile, is more likely to be determined by Congress than by Mueller. The special counsel is tasked with bringing any evidence he finds of criminal wrongdoing to Capitol Hill, at which point the question of Trump's fitness for office would become entirely political.

"The ultimate resolution of the Trump drama will be determined by political, not legal, forces," Seidman said. "Whether or not a pardon of, say, Kushner is an impeachable offense has little to do with the technicalities of contempt law."