This year I have been away from home a great deal working on a documentary series that will complement my forthcoming book on the history of women. The experience has been an eye-opener in many ways.

The past month, for example, has been spent in countries that don’t entirely share the BBC’s position on the bribing of public officials, or the European Union’s love of health and safety, or America’s belief in equality for all. What I witnessed made me feel lucky to be living in New York.

The airport may be a sorry dump but the rest of the city still sizzles with energy and optimism. Yet for the first time I have arrived back with a sense of foreboding.

Contrary to popular belief, democracies are not more robust than their totalitarian counterparts. It is in fact relatively easy to subvert a democratic institution from the inside, rotting the core while leaving the facade intact. Turkey, for instance, that beacon of Middle Eastern democracy, has the highest number of detained journalists in the world.

The creeping threat to the US political system is “lawfare”; not the type seen in Britain where illiberal organisations issue frivolous libel suits as a means of silencing critics and media outlets, but the kind that begins with a politically motivated investigation and ends with a jail sentence.

Earlier this month Rick Perry, the longest-serving governor of Texas and a Republican presidential hopeful, was indicted by a grand jury on two felony counts: abuse of power and coercion of a public servant. Together they carry up to 99 years in prison.

The Perry case rests on the flimsy premise that he abused his position by threatening to veto funding to a department of the Travis County district attorney’s office unless the district attorney resigned. Rosemary Lehmberg, the DA in question, had been sentenced to 45 days in prison for drink-driving.

The charges against Perry are so tenuous that even some Democrats are raising objections. David Axelrod, the former White House adviser and the current British Labour party guru, called the indictment “pretty sketchy”. He ought to know, having witnessed the relentless legal obstructionism endured by President Barack Obama.

First, it was the threat of impeachment. Now it is a congressional lawsuit initiated by John Boehner, Speaker of the House of Representatives, charging that Obama failed to carry out his constitutional duty with regard to the Affordable Care Act (the new healthcare system better known as Obamacare).

There is no possible chance of a court ruling in favour of the House. Moreover the suit labels as criminal what are simply political acts by the executive to improve some of the obvious deficiencies of the legislation.

Nor do the legal fun and games stop there. On the other side of the country Scott Walker, the Republican governor of Wisconsin — already the survivor of a recall vote organised by the state’s unions — has been under siege for two years while prosecutors have been trolling through his campaign finances looking for any evidence of illegality.

It is a sad fact of American politics that often there are good reasons for special prosecutors and grand juries to become involved. For a free and law-abiding country there is a surprising amount of criminality inside the corridors of power. The Washington Post once tried to count how many serving governors have ended up in prison; without a database to consult, its best guess was roughly 17.

The state of Illinois leads the pack with four out of the last seven governors jailed for corruption — and potentially a fifth if the federal probe of the current governor, Pat Quinn, leads to an indictment. West Virginia comes second with two governors turned felons.

Meanwhile, the governors of New Jersey and New York are also facing corruption investigations. Chris Christie of New Jersey, a Republican with presidential aspirations, is suspected of having colluded in the shutdown of an essential bridge lane simply to create a traffic nightmare for a local Democratic mayor.

Andrew Cuomo, a high-profile Democrat and possible presidential rival to Hillary Clinton, stands accused of trying to influence members of a special panel that he had personally appointed — oh the irony — to investigate the depth of corruption in New York politics.

With so many real corruption investigations, it has become all too easy for partisan officials to launch inquiries whose only purpose is to harass or eliminate political opponents. The legal attacks on Obama and governors Perry and Walker are merely the most prominent of their kind. Politics is being reduced to rule by fine print.

The definition of what constitutes an opponent is no longer confined to elected officials either. Campaign staff, fundraisers and donors are also fair game. The continuing Internal Revenue Service (IRS) “targeting” scandal is a case in point.

It is alleged that during the run-up to the 2012 presidential election, bureaucrats in the IRS’s tax exemptions department deliberately stalled the applications of conservative organisations. Even worse is the claim that outspoken Tea Party activists were subjected to tax audits. Lois Lerner, the former director of the exempt organisations division, has pleaded the fifth amendment against self-incrimination.

There was a time when Russia and China led the world in using legal thuggery as a political weapon. Unfortunately America is now catching up. In a slightly different context, the late French philosopher Jean-François Revel wrote in How Democracies Perish that free and open countries are the most vulnerable to manipulation.

Revel had a gloomy streak. He was none too sure of our survival: “Perhaps in history democracy will have been an accident, a brief parenthesis which comes to a close before our very eyes.”

But the use of lawfare as an alternative to fighting elections would have confirmed his worst fears. A democracy that is constantly waging a war with itself, Revel would have warned, is already halfway to defeat. America’s enemies only have to wait.