Sometimes history repeats itself in bizarre coincidences. Consider the intersection of the expected House vote tomorrow to begin a public phase of impeachment, and what happened 46 years ago today. Past and present may portend the future. On October 30, 1973, the House began a process to impeach President Nixon. The House Judiciary Committee voted along party lines to issue subpoenas and organize staffers. Four months later on February 6, 1974, the formal process was authorized in a nearly unanimous vote. Nine months later on November 5, 1974, the Republicans lost 48 seats in the House and four seats in the Senate.

History may not repeat itself and it may not stammer, but it is making Republicans nauseous. Despite the span of time between the Nixon and Trump impeachments, two political rules remain unchanged. First, you stick with the president of your party through thick and thin. Turning on him risks depressing your base. Second, you turn on the president of your party when things become too thick. Sticking with him for a day longer energizes the base of the opposing party and loses swing voters.

No one on Capitol Hill wants to end up on the wrong side of history and, perhaps more importantly, on the losing side of an election. That is why today, in private rooms and trusted conversations, the consensus among Republicans in Congress across the board is “who needs this?”

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To be fair, some of the stalwart defenders of President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Capitol Hill will never abandon him. When Trump said he could “stand in the middle of Fifth Avenue and shoot somebody” and not lose votes, he may have been referring to most Republicans in Congress who may have to vote on impeachment. But as the case against him grows more credible with witnesses such as Alexander Vindman Alexander VindmanImpeachment witness Alexander Vindman calls Trump Putin's 'useful idiot' The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Strzok: Trump behaving like an authoritarian MORE, now you can almost hear the painful turning of squeamish stomachs in the Senate.

It is not just that electoral sentiment could change. It is also that Trump seems to have a massive closet of other shoes to drop. Republicans just do not know what they do not know. What might pop by a credible witness in a hearing? What might the president himself or someone around him unwittingly confess? Acting White House chief of staff Mick Mulvaney already slipped on admitting quid pro pro with Ukraine.

The most worried man in Washington might be Senate Majority Leader Mitch McConnell, who makes political calculations every day. He must protect five Republican senators in potentially swing states. They are Susan Collins of Maine, Thom Tillis of North Carolina, Joni Ernst of Iowa, Cory Gardner of Colorado, and Martha McSally of Arizona. Some others have reportedly seemed at least open to voting against the president, including Mitt Romney of Utah and Lisa Murkowski of Alaska.

In a volatile and unstable political environment, the most useful strategic asset is the canary in a coal mine. This is why it all comes down to this critical correlation. The larger the number of House Republicans who vote to impeach Trump, the larger the number of Senate Republicans who will feel pressure to convict him. At this moment, the number might seem slim. It might have seemed the same way on October 30, 1973.