By Jan Wolfe

WASHINGTON (Reuters) - As Special Counsel Robert Mueller has pursued his investigation into Moscow's role in the 2016 U.S. election, legal experts have debated what sort of contacts between President Donald Trump's campaign and Russia may have violated U.S. criminal law.

U.S. intelligence agencies have concluded that Russia used a scheme of hacking and propaganda to cause discord in the United States and harm Trump's Democratic opponent Hillary Clinton. Mueller's probe, with several Trump campaign figures already pleading guilty or being convicted, has documented numerous contacts between Russians and people close to the president.

Mueller, who has sought to determine whether Trump's campaign coordinated with Moscow, is preparing to submit a report on his investigation to U.S. Attorney General William Barr. Trump and Russia have denied collusion and Moscow has denied election interference.

Here is a look at potential crimes Mueller may examine relating to these contacts and other matters.

IS THERE A U.S. FEDERAL CRIME CALLED COLLUSION?

Trump lawyer Rudy Giuliani in July 2018 said, "I have been sitting here looking in the federal code trying to find collusion as a crime. Collusion is not a crime." Trump wrote on Twitter the next day, "Collusion is not a crime, but that doesn't matter because there was No Collusion."

There indeed is no federal crime called "collusion." But collusion is a non-legal way of saying conspiracy, which is one of the most commonly asserted crimes in U.S. federal courts. Conspiracy is an agreement between two or more people to commit an unlawful act. A conspiracy does not need to have been successful, but the individuals must have taken some action to further it.

Because computer hacking is clearly a federal crime, any Trump campaign official who assented to and encouraged the hacking of Democratic National Committee computers in 2016 could be liable for the crime of conspiracy. U.S. officials have said Russia hacked the Democratic computers to steal emails that were later released by the WikiLeaks website to hurt Clinton.

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"Russia, if you're listening, I hope you're able to find the 30,000 emails that are missing," Trump said during a June 2016 news conference, referring to Clinton emails. "I think you will probably be rewarded mightily by our press."

The federal conspiracy statute also prohibits agreements aimed at "impairing, obstructing, or defeating the lawful function of any department of government," under a 1910 U.S. Supreme Court precedent.

This notion of criminal liability, known as conspiracy to defraud the United States, was raised by Mueller in a February 2018 indictment of 13 Russian individuals and three Russian companies, including St. Petersburg-based Internet Research Agency, known for its trolling on social media. According to the special counsel, their "information warfare against the United States" impaired the functioning of the U.S. Federal Election Commission and other government agencies.

Mueller in July 2018 also indicted 12 Russian intelligence officers accused of hacking the Democratic computers.

COULD TRUMP CAMPAIGN LINKS TO RUSSIA VIOLATE OTHER LAWS?

Yes. Campaign finance laws prohibit foreigners from influencing U.S. elections, and presidential candidates cannot legally solicit campaign contributions from foreign nationals. Campaign contributions are defined broadly as anything of value intended to influence the election.

The hacked Democratic emails potentially could fall into that category, if Mueller finds coordination between the Russians and Trump's campaign.

If Trump campaign officials knowingly solicited valuable information from Russians, they may be liable under campaign finance laws, legal experts said. But Trump campaign officials would have a strong defense if they could show they did not know soliciting such help violated campaign finance laws. While ignorance of the law often is not an excuse, criminal violations of campaign finance law are unusual in that they require a showing of willfulness, or knowledge of the law being broken.

WHAT OTHER CHARGES MIGHT MUELLER CONSIDER?

Mueller also is looking into whether Trump engaged in obstruction of justice by trying to impede or shut down the Russia investigation, first when it was conducted by the FBI and later after the special counsel was appointed in May 2017. There are several federal laws that make it a crime to interfere with a court case or government proceeding.

One broadly worded federal law prohibits efforts to "influence, obstruct, or impede the due administration of justice." The effort can be unsuccessful and still be obstruction. Obstruction of justice cases are often difficult to prove because they hinge on an individual's mental state. Prosecutors would need to show that Trump acted with a "corrupt" intent, or an intent to impede an investigation.

According to some lawyers, Trump engaged in obstruction of justice by asking former FBI director James Comey in February 2017 to back off an investigation into U.S. national security adviser Michael Flynn over Flynn's contacts with Russia. Trump eventually fired Comey in May 2017.

Other lawyers have said the obstruction of justice case against Trump is weak because the president could credibly argue he did not actually pressure Comey and that he fired him for reasons unrelated to the Flynn investigation. Trump himself has given differing explanations for the firing, including citing "this Russia thing" as the reason. Some legal scholars have also argued that the president cannot obstruct of justice by exercising his constitutional authority to fire a subordinate.

Under current Justice Department policy first devised in 1973 and reaffirmed in 2000, a sitting president cannot face criminal charges, a view some legal experts dispute. But any accusation against Trump of conspiracy, obstruction or justice or other crime could prompt an impeachment effort in the U.S. Congress to remove him from office. The U.S. Constitution states, "The president, vice president and all civil officers of the United States shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors."

(Reporting by Jan Wolfe; Editing by Will Dunham)