"The work of the special counsel is not a substitute for an impeachment inquiry. The special counsel investigates criminal acts. The House, which has the sole constitutional authority for impeachment, would consider more broadly any abuses of presidential power that constitute impeachable offense, explicitly criminal or not."

The Committee voted 33 to 3 to subpoena the president's White House tapes and ultimately more than a third of committee Republicans voted for at least one article of impeachment against Nixon. The president resigned after the leadership of the Republican party told him he was virtually certain to be impeached by the full House and convicted by the Senate.

Of course, political calculation also figured into the Republicans response to the Watergate scandal. The Republicans lost 4 of 5 special U.S. House elections in 1974. They lost the seat in Grand Rapids Michigan that appointed vice president Gerald Ford had held for decades.

In April 1974, after the GOP lost another Michigan House seat that the party had controlled since the 1930s, reporters asked respected Republican Senator Charles H. "Chuck" Percy of Illinois whether President Nixon should resign. "Probably so," Percy replied. "It probably would be somewhat advantageous to the Republican Party and the country." Nixon resigned some three and a half months later, on August 8, 1974.

Nixon's resignation was good for the nation. It removed a president who threatened American democracy and the integrity of its governmental institutions. It was also good for the Republican Party. The GOP lost 49 House and four Senate seats in the midterm election of 1974. But their losses were not much greater than the norm for the party of the president in a midterm election. The Republicans came roaring back with big victories in the midterm election of 1978 and with Ronald Reagan's landslide defeat of President Jimmy Carter in 1980.

Republicans today would be well advised to follow the Watergate precedent and vote to authorize an impeachment investigation of President Donald Trump by the House Judiciary Committee. It is in the interest of the nation and the Republican Party to conduct a thorough assessment of the allegations against President Trump no matter where the facts may lead.

The work of the special counsel is not a substitute for an impeachment inquiry. The special counsel investigates criminal acts. The House, which has the sole constitutional authority for impeachment, would consider more broadly any abuses of presidential power that constitute impeachable offense, explicitly criminal or not. Unlike the Congress, the special counsel works in secret and is accountable only to Trump's Department of Justice, not the American people. During Watergate, we had simultaneous investigations by a special prosecutor, the Senate Committee, and the House Judiciary Committee.

As in 1974, this year's special elections hold a warning for the Republican Party. The Republicans barely held on to overwhelmingly Republican U. S. House seats in in Kansas and Montana. They could well lose the June 20 special election for a Georgia House seat that Health and Human Services Secretary Tom Price won by 23 percentage points in 2016 and was formerly held by Newt Gingrich. Republicans hold scores of other U.S. House seats in which they are more vulnerable to defeat than in this Georgia district.

As in Watergate, Republicans need to rise to the historical moment and put patriotism above party. If what Trump says is true, that he did nothing wrong as candidate or president, then he and every fellow Republican should welcome an impeachment investigation as the only sure way to dispel the clouds of controversy hanging over the Trump presidency.

Commentary by Allan Lichtman distinguished professor of history at American University in Washington, DC and author of the new book The Case for Impeachment. Lichtman is known for a model he developed that has accurately forecast the outcome of every presidential contest since 1984, including Trump's recent win. Follow him on Twitter @ AllanLichtman.

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Correction: Under Article 2, Section 4 of the Constitution, a vice president no less than a president is subject to impeachment. An earlier version of this op-ed misstated the article and section of the Constitution.