In Watergate, the smoking gun was a White House tape proving that Richard M. Nixon ordered a cover-up — the final evidence that forced him from the White House.
In the investigation of Russian interference in the 2016 presidential election campaign, the smoke hasn't cleared because President Donald Trump keeps shooting.
There may never be a smoking gun situation in this case, despite the inevitable comparisons between the Trump-Russia investigations and Watergate.
As someone who covered Washington in the early 1970s, I can tell you: For any parallels, there are also major differences.
Watergate began, clearly, with a crime: the break-in at Democratic Party headquarters in June 1972. There is no such evidence of a crime in the Russia case, although Trump's Democratic detractors suggest investigations might find obstruction of justice.
They cite ex-FBI Director James B. Comey's reported account of a conversation in which Trump asked him to lay off the case of ousted National Security Adviser Michael Flynn over Flynn's Russia contacts, saying, "I hope you can see your way clear to letting this go." Later, of course, he fired the FBI chief — denying that he did so to impede the Russia investigation.
But there has been nothing comparable to the tape in which Nixon agrees with a top aide to tell the FBI to "stay the hell out of this." Three days after that tape was released in August 1974, Nixon resigned rather than face impeachment.
The Trump White House has effectively confirmed The New York Times account of his statements to Russian officials —that Comey was "a real nut job" and that ousting him had released great pressure he'd faced because of Russia — but insists they are being misinterpreted by his opponents. With that, and in this whole episode, they fall back on a tactic Nixon and his aides used in Watergate: Attack the disclosure of classified information rather than deny the substance. Since the administration controls what is classified, that is an all-purpose umbrella.
The Comey firing and what Trump has said since recall the Saturday night massacre, when Nixon dismissed Watergate special prosecutor Archibald Cox over Cox's subpoenas for White House tapes - the evidence that would eventually prove the cover-up.
Cox had been appointed in the first place because the Senate demanded a special prosecutor before it would confirm Nixon's new attorney general, Elliot Richardson. Richardson promised not to interfere with the prosecutor, and when Nixon ordered him to fire Cox, Richardson resigned. So did his deputy, before Nixon got Cox out and tried to abolish his whole operation. The president failed — Congress and public pressure prevented it.
A new special prosecutor stayed on the job, his subpoenas were upheld by the courts, and Nixon was finished.
So the precedents were not promising when Trump fired Comey.
Trump's action has not ended the investigation. It has, if anything, intensified it. Now there is a special counsel on the case, appointed by Deputy Attorney General Rod Rosenstein and bitterly denounced by Trump, who called it "the greatest single witch hunt of a politician in American history."
All of this has created an atmosphere not unlike the uncertainty and apprehensions of the Watergate era.
In Nixon's final months, top Cabinet officers feared that he would order military action abroad to ease his plight at home — or even to hold power. They told military leaders not to order action on Nixon's command without checking with them first.
Now there is another season of mistrust, although it has not sunk to Watergate lows.
And Trump has a political advantage Nixon did not. Trump's approval ratings are low, in the 40 percent range, but many of the people who voted for him are not moved by the controversies.
In some ways, he is not constrained by the rules that bind a more traditional leader. Nixon was a product of the establishment and the Republican Party— he'd risen through Congress and the vice presidency. So when Republicans turned against him in Watergate, it signaled the end.
That is less threatening to Trump. He can denounce and ignore defectors, and his loyalists will applaud the man they elected to break through — and break up — the ways of Washington government.
The real risk is in the investigations, the special counsel, the Senate and House committees probing Russian meddling and, perhaps, the courts.
Should special counsel Robert Mueller, who served as FBI director for 12 years, push the Russia investigation to a point that threatens — or angers — Trump, the president has the power to fire him, too.
That would come with a price — a political explosion comparable to outrage over the Saturday Night Massacre. But the country is far more sharply divided than in Nixon's time.
The Democrats who talk about impeaching Trump ignore that and the numbers. Democrats were in charge when Nixon faced the impeachment charges that led him to resign. A Republican House impeached President Bill Clinton before the Senate acquitted him for lack of the required two-thirds vote.
So impeachment talk in this case is not only premature. Impeachment is politically impossible with Republicans in firm House control, barring disclosures far more explosive and incriminating than anything to date.
That said, the investigations and the controversy are going to go on, and Trump is sure to remain on the attack with his tweets and use of non-stop cable television, which did not exist during Watergate.
When Trump was asked whether he'd tried to disrupt the Russia investigation, he snapped, "No, no, next question."
There will be many next questions.
Early in the Watergate investigation, Nixon declared that he would "never, never, never, never" again discuss the case. He did, of course. He had no choice.
Just as that case beset Nixon, so will this one nag Trump. And the president is not a man who yields to nagging in silence.
This story has been corrected to reflect that Michael Flynn's title was national security adviser.