Donald Trump’s performance at his meeting with Vladimir Putin in Helsinki has convinced many observers that the US president has crossed the line into overtly colluding with a foreign power against the United States. As Trump parroted Russia’s line over that of his own intelligence agencies, the former CIA director John Brennan even said that his actions were “nothing short of treasonous”.

For all his denials of collusion – and his later back-pedalling – Trump himself publicly asked Russia to hack into Hillary Clinton’s emails during the presidential campaign. And according to a new indictment handed down by Robert Mueller, Moscow’s agents attempted to do just that for the first time later the same day.

Has any other presidential candidate so brazenly sought to influence a US election with the aid of a foreign power? Actually, yes. In the 1968 election, Richard Nixon sought to collude with the government of South Vietnam to undermine a peace plan being pushed by his Democratic opponents to end the Vietnam war. Worried that the plan’s success might lead to the election of Nixon’s opponent, Hubert Humphrey, Nixon reached out to South Vietnam’s government telling them to reject the deal and wait for better terms under a future Nixon administration. South Vietnam held out, Nixon was elected by a narrow margin, and the Vietnam war ground on for four more years.

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News of Nixon’s extracurricular diplomacy reached Lyndon B Johnson’s White House in the closing weeks of the election campaign. Nixon’s intermediary to South Vietnam, Anna Chennault, was recorded on an FBI wiretap telling its government to “hold on” because “we are gonna win”. Johnson, distraught at the continued carnage in Vietnam, flew into a rage when he read the transcript. He even called it “treason”.

Yet Johnson’s reaction revealed a design flaw in American democracy that unscrupulous politicians like Nixon or Trump can exploit. Lacking any smoking gun linking Chennault’s actions to Nixon, Johnson ultimately decided not to go public. Like Barack Obama in the autumn and summer of 2016, when reports of Russian election interference reached the White House, Johnson decided that doing so might look like an attempt to use the covert apparatus of the state to unfairly discredit his opponent. In both cases, politicians who scrupulously adhered to democratic norms lost out to those who did not.

Much of the fear and speculation that now surrounds Trump concerns how he would react in a similar situation as president. In the run-up to this year’s midterm elections, US cyber-intelligence officials report that Russia is continuing efforts “to undermine our democracy” and that “the warning lights are blinking red again”. Trump’s performance in Helsinki places the question of whether he will act appropriately on these warnings – pushing back against an interference campaign that aims to benefit him politically, just as it did in 2016 – in grave doubt.

Yet Nixon’s ultimate fate should give us hope. Although the “Chennault affair” largely passed into the history books, Nixon was eventually felled in the Watergate scandal thanks to a combination of zealous intelligence and law enforcement work and a belligerent and insistent free press. While the popular myth of the Watergate scandal accords most of the responsibility for Nixon’s downfall to the media, in reality its role was more to publicise the work being done by the FBI, prosecutors and Congress. Without these institutions, Watergate would have gone nowhere.

We can now understand why Trump, like all autocrats, attacks both the media and independent centres of power in what he labels the “deep state” so relentlessly. Even liberal observers are right to be uncomfortable with the prospect of the FBI and the spooks bringing down a president. But, as we saw during Watergate, these institutions are important parts of the system of checks and balances that can sniff out illegal behaviour and even – yes – “treason”. This is why Trump attacks them. And it is why we must defend them.

Whether Trump’s performance in Helsinki was treasonous or rose to the level of the “high crimes and misdemeanours” that the US constitution says warrants impeachment is open to legal and political debate. In the coming months and years, as the Trump administration responds to Russian cyberattacks and other menaces, there will be plenty of officials in US intelligence and law enforcement institutions with a privileged insight into this question. They will have to look to their consciences to decide how to act. The fate of America’s last colluder, Richard Nixon, gives us hope they will choose wisely.

• Andrew Gawthorpe is a lecturer in history and international studies at Leiden University