WASHINGTON – It started with the White House going after a political opponent and ended up with a disgraced president being forced from office.

It might be too early to invoke the specter of the Watergate scandal that cost President Richard Nixon his job 45 years ago. But President Donald Trump's pressure on Ukraine to investigate 2020 presidential challenger Joe Biden is increasingly drawing comparisons to one of America's darkest chapters.

Trump's efforts to lean on Ukrainian President Volodymyr Zelensky is reminiscent of the way Nixon created a team of secret investigators, known as "the plumbers," to find incriminating or embarrassing evidence about his enemies, said Ken Hughes, a leading Watergate authority and research specialist at the University of Virginia’s Miller Center of Public Affairs.

"The difference between Nixon and Trump is that, for Nixon, the plumbers' operation was run and staffed by Americans, but Trump is outsourcing the dirt-digging operation overseas," Hughes said. "So it's actually shockingly similar."

The comparisons are flowing more frequently since Wednesday's release of a summary detailing Trump's July 25 call to Zelensky and Thursday's release of a whistleblower complaint that the administration had taken steps to cover up details of the phone conversation.

On the call, Trump reminded Zelensky that "we do a lot for Ukraine" before asking the Eastern European leader for "a favor." Later in the call, Trump asks that Zelensky talk with his personal lawyer, Rudy Giuliani, the former New York City mayor who has been investigating the activities of Biden and his son, Hunter, who was on the board of a Ukraine energy company.

The revelations prompted House Speaker Nancy Pelosi, D-Calif., to launch an impeachment inquiry Tuesday and accuse Trump of betraying his oath of office and endangering national security.

Trump has emphatically denied that he applied any pressure on Ukraine. And he lashed out Thursday at the unnamed officials who provided the whistleblower with details of his phone call, calling the source of the leak “almost a spy” and suggesting the culprit had committed treason.

“You know what we used to do in the old days when we were smart with spies and treason, right?” Trump said, according to published reports. “We used to handle it a little differently than we do now.”

The call:'I would like you to do us a favor.' What Trump and Zelensky said in their July 25 phone call

The complaint:Read the full declassified text of the Trump whistleblower complaint

The two-year scandal that took Nixon down began when a security guard discovered that five men, later linked to the president, had broken into Democratic National Committee headquarters at the Watergate Office Building in Washington on June 17, 1972, in an attempt to plant listening devices.

The Watergate scandal unveiled to the American public the president's clandestine efforts to punish political enemies and his subsequent attempts to cover up inappropriate conduct.

Paul Rosenzweig, a senior fellow at the R Street Institute and a former Whitewater prosecutor under Kenneth Starr, said there's a strong parallel between what happened more than four decades ago and what Trump is doing.

“Nixon’s problems stem from using presidential authority to protect himself and advance his own political interest. … Trump’s activities sound a lot more like that. It’s using governmental authority to advance his own interest," Rosenzweig said.

Rosenzweig, though, said the allegations against Trump are highly unprecedented.

“We’ve never had a president blatantly use foreign powers,” he said. “Nixon was all about domestic stuff.”

Nixon resigned from office in August 1974 before he could be impeached as Republicans in Congress began turning on him. Most Republicans on Capitol Hill appear to firmly back Trump and have discounted Democratic criticism of the president's conduct as an effort to undo Trump's 2016 victory.

"As to the whistleblower complaint – the transcript speaks for itself – no quid pro quo. The Democrats bought a pig in a poke," Sen. Lindsey Graham, tweeted Thursday.

Asked about the comparisons to Nixon during an interview Tuesday with The Atlantic's editor-in-chief Jeffrey Goldberg, Pelosi said it's hard to ignore.

"Let me just say, in terms of the Nixon analogy, the president is making lawlessness a virtue in our country," she said. "In that call, he sort of is trying to export it."

Ross Baker, a political science professor at Rutgers University in New Jersey, suggested the comparison is probably unfair – to Nixon.

Leon Panetta:Former CIA Director: 'Lock down' of Trump's Ukraine call a sign 'they were at least thinking of a cover-up'

"Nixon was less devious. Republicans were saner. Democrats were bolder. Congress was less intimidated by presidents. Journalists were less distracted by shiny objects. And Americans had better standards for who they picked as president," he wrote in an email.

Patrick Cotter, a former federal prosecutor in New York, said that in the gamut of presidential conduct that has led to impeachment proceedings, the allegation against Trump “is very close” to those leveled against during the Watergate scandal.

“The parallels are very clear,” Cotter said, citing the effort to gain advantage against a political rival and covering it up in a way that compromises the integrity of the judicial process and the integrity of the election.

The complaint reports a "repeated abuse of an electronics record system designed to store classified, sensitive national security information, which the White House used to hide information of a political nature."

White House officials told the whistleblower that they were "directed" by White House lawyers to remove the electronic transcript of the call from the computer system that stores them for coordination, finalization and distribution to Cabinet-level officials, according to the complaint.

Officials also said that the steps taken to protect the contents of the calls were not the first time that a transcript of Trump's conversation was put into a code-word-secured system solely to protect information that was politically sensitive rather than a national security concern, according to the whistleblower.

"Whatever other explanation there might be, an explanation is also that it is a cover-up," Pelosi told reporters Thursday using a phrase long associated with the Watergate investigation.

Hughes, the University of Virginia expert on Watergate, called Trump's conduct involving Ukraine "Nixonian."

"It's clearly an abuse of power and abuses of power have traditionally been impeachable acts," he said. And "it's clear that Trump, like Nixon, is not being honest about what he was up to."

But whether he'll be forced from office like Nixon remains in the hands of Republicans who must first run in GOP primaries full of loyal Trump voters if they want to win reelection next year.

Hughes said the "political calendar" was one reason Republicans stuck with Nixon as long as they did despite the mounting evidence against the president.

By August 1974, Republican lawmakers had already wrapped up their nominations so they could abandon the president because they no longer worried about offending their base.

"They were heading into the general election so they were worried about offending the middle, and the middle had turned against Nixon by August of '74," he said. "Republicans in Congress realized that he was going to drag them down if he remained in office. That made all the difference."

Contributing: Kristine Phillips, Michael Collins and Maureen Groppe