CHICAGO (CBS News) — House Speaker Nancy Pelosi on Tuesday announced the opening of an official impeachment inquiry against President Donald Trump.

The tipping point was reports that Trump pressured the president of Ukraine to investigate Joe Biden, a Democratic candidate for the presidency.
President Trump acknowledged Sunday that he discussed Biden in a phone call with the newly elected president of Ukraine in July. One week before that call, Mr. Trump had instructed aides to hold off on releasing nearly $400 million in military aid to Ukraine, a senior administration official confirmed to CBS News.

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Impeachment is the constitutional process where the lower house of legislative branch — the House of Representatives — brings charges against a civil officer in government — in this case the president. It is similar to an indictment, and the Senate then tries the case with the chief justice of the Supreme Court overseeing the trial. A two-thirds majority in the Senate is required to convict and remove the president from office. That means 66 senators would need to agree to remove Trump.

Only two presidents have previously been subjected to impeachment proceedings.

In December 1998, the House introduced and approved articles of impeachment against President Bill Clinton. He faced a Senate trial for two charges: perjury and obstruction of justice. The allegations stemmed from his relationship with White House intern Monica Lewinsky.

President Clinton had denied a sexual relationship with Lewinsky in his a sworn deposition in January 1998, followed days later by a press conference in which he infamously declared: “I did not have sexual relations with that woman.” He also denied other accusations that he urged Lewinsky to lie about the affair.

But an investigation by independent counsel Ken Starr concluded that Clinton lied during the deposition. In August 1998, Clinton admitted to having a relationship with Lewinsky, and the House voted to hold an impeachment inquiry against Clinton in October.

The impeachment trial in the Senate commenced in January 1999. Ultimately, with 45 “guilty” votes on perjury and 50 “guilty” votes on obstruction in the Senate, Clinton was acquitted and remained in office.

Previously, some Democratic leaders – including Pelosi – had been wary of proceeding with articles of impeachment against Trump. Some cited the backlash following the Clinton impeachment as a reason.

Republicans faced political backlash for their efforts at the time, losing congressional seats in the 1998 midterm and seeing Clinton’s approval rating rise as a result. Some Democratic leaders argued that with the 2020 presidential election already getting under way, elections should decide the future of the commander in chief.

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In 1868, President Andrew Johnson was impeached after running afoul of so-called Radical Republicans in Congress amid Reconstruction after the Civil War. The prior year, Congress passed the Tenure of Office Act, which gave the Senate the right to approve or reject the dismissal of all presidential appointees. In drafting that law, the Radical Republicans set a trap for Johnson – and he soon fell into it.

To challenge the Tenure of Office Act, which he regarded as blatantly unconstitutional, Johnson fired his Secretary of War, Edwin Stanton, who was closely allied to the Radical Republicans. But instead of initiating a Supreme Court case on the question, which had been his intention, Johnson gave the Radical Republicans the ammunition they needed to launch impeachment proceedings against him.

For Johnson, the vote to impeach in the House was 126 to 47.

At the time there were 42 Republicans in the Senate and only 12 Democrats. If the vote had been strictly along party lines (as had been the case in the House), there would have been more than enough to meet the two-thirds majority needed to convict the president. But when push came to shove, seven Republicans joined with the Democrats, and Johnson was acquitted by the perilous margin of one vote.

It was later disclosed that just enough Republicans had secretly pledged to vote for acquittal because they believed that to remove a president over what were essentially policy disagreements and political power moves would set a dangerous precedent for the future.

Impeachment proceedings were also held against President Richard Nixon in 1974 – but he resigned from office before any articles of impeachment could be approved.

Nixon and his top deputies had been furious over the publication of the Pentagon Papers regarding the Vietnam War, and Nixon had approved the formation of a Special Investigations Unit with a mission was to prevent and plug leaks that might be damaging to the White House.

The members of this task force were the infamous “Plumbers” who, in time, would go on to burglarize Democratic campaign offices in Washington and commit other crimes that fell under the rubric of Watergate. And when Nixon made the fateful decision to take part in the cover-up of these crimes, he put his presidency squarely on the road to destruction.

The articles of impeachment against him were drafted in late July 1974, and a few days later a delegation of Republican leaders on Capitol Hill visited the White House. There, in somber tones, they managed to persuade Nixon that he not only would be impeached on the House floor but that he faced certain conviction in the Senate trial that would follow the House action.

So, rather than go through that ordeal, he became the first president in American history to resign from office.

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