Sarah Isgur is a CNN political analyst. She has worked on three Republican presidential campaigns and is an adjunct professor at George Washington University's School of Media and Public Affairs. She is a graduate of Harvard Law School. The views expressed here are the author's. Read more opinion on CNN.

(CNN) In 1972, President Richard Nixon's administration was plagued by scandal. But not the one you're thinking of.

This was the ITT scandal. And President Donald Trump's White House would be wise to read up on it, because it just might explain the recent and sudden movement in national polls toward impeachment.

I was reminded of the ITT scandal when I noticed Trump's call with Ukrainian president Volodymyr Zelensky had come the day after Special Counsel Robert Mueller testified before Congress about the Russia inquiry. And then I thought about how quickly current poll numbers on impeachment are moving now, given that we saw no polling bump after Mueller's testimony.

There are many differences between the 1970s and now. But it's possible that in Nixon's case, the initial corruption allegation around the ITT scandal may have softened the ground for impeachment by the time Watergate came around -- even if the ITT scandal didn't move poll numbers at first.

If that's true, it's also possible that voters simply reach a tipping point on presidents like Nixon and Trump who can't help but run back into the fire just days after almost being burned.

In 1971, the Department of Justice was conducting an independent investigation into ITT, the technology manufacturing company founded as International Telephone & Telegraph, when Nixon told then-Deputy Attorney General Richard G. Kleindienst not to appeal one of ITT's suits to the Supreme Court.

It was no coincidence that, around the same time, the DOJ's target in the investigation assisted the Republican National Committee and the President's campaign in a big way : ITT contributed $400,000 (roughly $2.5 million in today's dollars) to the Republican National Committee for the GOP's 1972 convention.

The deal came to light about a year before Nixon's reelection. While some observers suspected there had been some sort of quid pro quo arrangement, it wasn't until a still-unknown whistleblower sent an internal memo to a syndicated columnist that things heated up. As if to emphasize the document's shadiness, the memo ended with the line: "please destroy this, huh?"

Brit Hume , then just a scrappy young reporter working under syndicated columnist Jack Anderson, confirmed the memo's authenticity in the Washington Post, saying he'd gotten confirmation from ITT spokesperson Dita Beard. (Beard later denied that she'd offered Hume this confirmation.) Vice President Spiro Agnew was involved in the affair, holding meetings with ITT officials.

But Nixon was unchastened and persisted in his quest for reelection by whatever means necessary. His approval numbers sagged, but his support among Republicans actually increased in the early months of 1972. In fact, as the ITT scandal unfolded that spring, most polls were still predicting the President would win reelection.

The Democrats had over a dozen serious candidates vying for the nomination that year -- their most diverse field in history. More than a year out, the candidate leading the pack with an average of just under 30% in Democratic primary polls was Senator Ed Muskie, an East Coast moderate who appealed to blue-collar voters. The White House viewed Muskie as the biggest threat to the President's reelection; in early 1972, the two were tied in polls.

In response, Republicans peddled false attacks against the Democratic frontrunner, including attacks against Muskie's family. Muskie's pushback was seen as too weak in the press, and his lead began to slip. The Democrats then looked to their progressive base for a nominee.

In the meantime, Democrats in Congress were still investigating that ITT memo and sending out document requests without much success. But Nixon -- paranoid, obsessed with winning and undeterred by the continuing inquiry -- jumped back into the fire with both feet by participating in a coverup after his campaign orchestrated a break-in at the DNC headquarters at the Watergate Hotel to spy on the opposition's operations.

And this time, his deeply inept team got caught.

Nixon's most loyal supporters in the conservative media downplayed the Watergate allegations at first: "I can see no grounds for impeachment , or even to get worked up about," said conservative publisher Henry Regnery. Or else they blamed the deep state, as Regnery did when writing to a friend: "[This scandal] clearly demonstrates that the press and the bureaucracy, working together, can destroy the president."

But public opinion in favor of impeachment started to creep upward the following year. The President talked about it endlessly at press conferences . Two and a half years after that initial "please destroy this" memo came to light, the President was out of a job.

While the special prosecutor's definitive report in the ITT scandal didn't find evidence of criminal conduct, the scandal, though quickly forgotten, may well have laid the foundation for Nixon's resignation.

9th August 1974: U.S. President Richard Nixon looks down as stands at a podium, reading a farewell speech to his staff following his resignation, the White House, Washington, D.C. (Photo by George Tames/New York Times Co./Getty Images)

Here's the transcript from Nixon's White House tapes of a conversation between Nixon and his Deputy Attorney General Richard G. Kleindienst in the late spring of 1971:

Nixon: "The I.T.T. thing — stay the hell out of it. Is that clear? That's an order .... I do not want [Assistant Attorney General Richard] McLaren to run around prosecuting people, raising hell about conglomerates, stirring things up."

After a little more back-and-forth, Kleindienst says ,"Yeah, I understand that."

The ITT affair only became a scandal for Nixon when a reporter anonymously received the memo, which appeared to show that ITT had offered financial support for the convention in exchange for the DOJ dropping the lawsuit. (A special prosecutor found no evidence of wrongdoing by ITT.)

The story blew up in early 1972, but Kleindienst, who by then had been nominated for Attorney General, testified under oath that the White House never asked him to intervene in the case.

Polling at the time showed that 59% of Americans were following the ITT scandal and that only 24% believed Kleindienst. Even so, Kleindienst was confirmed by the Senate on June 8, 1972, and ITT faded into the background as "a setback for the Nixon Administration...[but not] the political bombshell some Democrats hoped it might," as Lou Harris wrote in a polling memo at the time.

Once Nixon had dodged the ITT bullet, his reelection seemed all but assured, as he enjoyed a 20-point lead in the polls heading into the summer of 1972.

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