Donald Trump is facing the first major threat of his young presidency as pressure mounts for a systematic probe into the contacts between his staff and Russian officials at a time when Russia was attempting to influence the U.S. election.

Mr. Trump's imbroglio carries echoes of controversies that have hobbled some of his predecessors. Those previous scandals hold both warning and comfort for the 45th U.S. president: only once have investigations actually ended a president's term, but in many cases, they have sapped the energy and efficacy of the office.

Richard Nixon speaks at a White House news conference in the midst of the Watergate investigation, in 1973. CHARLES TASNADI/AP

Watergate: Don't break the law

For some who lived through past crises, the reverberations are hard to ignore. Asked if he heard echoes of the Watergate scandal in the current controversy, John Dean – a former White House counsel to president Richard Nixon – laughed and said "absolutely." Mr. Dean said Mr. Nixon also demonstrated an obsession with leaks and, in private, a tendency toward authoritarianism.

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Watergate sits at the apex of all presidential scandals. In 1972, a bungled break-in at the headquarters of the Democratic National Committee led to a cover-up by the White House, which became the subject of an expanding investigation.

Two years later, legislators considered impeaching Mr. Nixon for obstruction of justice, abuse of power and contempt of Congress. On Aug. 7, 1974, leaders from his own party told Mr. Nixon that he had lost the support of Congress and faced near-certain impeachment. The following day, he resigned.

"The big difference is that everything moved at a slower pace during Watergate" because it unfolded in an era before the Internet and social media, Mr. Dean said. "If there are any lessons from Watergate, it's transparency and obeying the law."

One dynamic working in Mr. Trump's favour is the fact that his party holds majorities in both the House of Representatives and the Senate. In periods when there is "unified" party control of the White House and Congress, investigations are less frequent and aggressive. By contrast, when there is divided control, one party often uses its command of a legislative body to launch probes to embarrass or discredit the president.

There is already an investigation by the U.S. Federal Bureau of Investigation into Russia's tampering in the U.S. election and into ties between Mr. Trump's associates and Russian officials. The Senate and House Intelligence Committees, both controlled by Republicans, have also said they will investigate. But Republicans have pushed back against calls by Democrats for a more muscular approach, for instance by forming a special joint House-Senate committee or by appointing an independent prosecutor.

Republicans involved in the investigations are "going to walk slowly, they're going to be very careful, they don't want to make mistakes," said James Thurber, a political scientist at American University and founder of its Center for Congressional and Presidential Studies. But if Mr. Trump "continues to do things that so obviously undermine national security and trust in government, they have to move."

Ronald Reagan (right) is seen with Caspar W. Weinberger in 1987. In his role as defense secretary, Weinberger became a central figure in the Iran-Contra scandal. JOSE R. LOPEZ/NYT

Iran-Contra affair: Strategic disclosure

Watergate is the exception to the rule. More often, presidents weather scandals but find themselves consumed in a struggle to limit the damage.

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During president Ronald Reagan's second term, it emerged that the administration was involved in a scheme to sell weapons to Iran to help secure the release of hostages, while using the proceeds of the sales to fund right-wing militants known as Contras in Nicaragua. Both parts of the scheme were illegal under existing law.

Weeks after news of the weapons sales emerged in November, 1986, Mr. Reagan announced the creation of a three-member presidential commission to explore what had happened. In the ensuing report, Mr. Reagan "took some bumps and bruises" for being inattentive, noted Craig Shirley, a Reagan biographer and presidential historian, but the blame for the plan was placed squarely at the feet of his aides.

Mr. Trump would be wise to do something similar, Mr. Shirley said. "If he created his own commission to investigate the real story behind [former national security adviser] Michael Flynn and the leaks, he could take control of the politics," he said. "If he believes that Flynn was undone by illicit leaks by the CIA and the FBI, then he has nothing to fear."

Ultimately, 14 Reagan administration officials faced criminal charges in the Iran-Contra affair, including the secretary of defense and the national security adviser. All were later pardoned by President George H.W. Bush, Mr. Reagan's successor and former vice-president.

Bill Clinton and Monica Lewinsky at the White House in 1995. AP/OIC

Lewinsky affair: don't lie

Another major presidential scandal exploded in early 1998 involving Bill Clinton. Mr. Clinton had repeated sexual encounters with Monica Lewinsky, a White House intern, and later denied those incidents in legalistic terms when questioned in a separate lawsuit.

In December, the Republican-controlled House of Representatives voted to issue articles of impeachment for Mr. Clinton for perjury and obstruction of justice. But after a 21-day trial, the Senate, also controlled by Republicans, ultimately acquitted Mr. Clinton, leaving him in office.

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Mr. Clinton became only the second president in U.S. history – after Andrew Johnson in 1868 – to experience an impeachment trial. And while he survived it, his position and legacy were diminished.

"When Clinton lied to his staff, lied to his wife and lied to America, it really undermined his capacity to govern," Prof. Thurber said. Fighting the scandal burned up much of the political capital Mr. Clinton might have used to achieve other goals in his second term, he added.

Prof. Thurber said that Mr. Trump, who already has demonstrated a loose commitment to telling the truth, should recall that what often hurts presidents is not the scandal so much as the lying about it. "Ultimately, there are no secrets in Washington," he said. "Sometimes it takes years, but ultimately they come out."

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