Speaker Nancy Pelosi departs after speaking to the media before presiding over the House vote on a resolution formalising the impeachment inquiry © Erik S Lesser/EPA

A capital as partisan as Washington does not need an issue of substance to divide it. A matter of process will do. The resolution that passed the House of Representatives last week was a case in point. It did not seek to impeach Donald Trump for pressing Ukraine to investigate a political rival. It did not even frame what the articles of impeachment against the president might be. Rather, it set out how Congress would proceed with its inquiry. It dealt with rules rather than content.

Still, all but two Democrats voted for it while no current Republican did the same. As both a precis of recent decades and as a trailer for what is to come, the resolution, which passed along these party lines, was grimly accurate.

As rancorous as the following weeks will be, the resolution also contains some cause for hope. Until now, the inquiry into Mr Trump has played out mostly in private. It is about to break into the open. The intelligence committee of the House is to hold public hearings. Depositions taken in closed quarters could be published. There is the prospect of sustained and televised questioning of witnesses.

As such, it will be harder for Republicans to slam the inquiry as an establishment plot. A “deep” state that acts in public, as global audiences watch, cannot be all that deep. For their part, Democrats will also feel an onus to hone their lines of attack. There will be histrionics and grandstanding — this is Washington — but also opportunities for public scrutiny.

Privacy has been essential until now to ensure the candour of witnesses. When the alleged abuse involves foreign policy, rather than a burglary or an affair, there are security implications, too. The trouble is that privacy is also fuel for conspiracy theorists. It gives the electorate only partial information.

It is doubly a problem when voters are asked to trust the unseen diligence of Congress — the least trusted institution of note in the country.

Comparisons with impeachments past are always fraught. The sample size is small and America is not the country it was in 1998, let alone 1868. But at least one lesson from the impeachment (and subsequent acquittal) of Bill Clinton is instructive here.

In going after him, the Republican Congress learnt more from a dense report by Ken Starr, compiled in private over several years, than from open hearings. This contributed to a sense of presidential ambush that put off many potentially neutral observers.

It also led to confusion about the issues at hand that survives to this day. Many still mistakenly believe that Mr Clinton was impeached for an affair with Monica Lewinsky. A more public process would have been clearer for all its ugliness. Robert Mueller’s report into Russian meddling in US politics was another case of (necessary) discretion leading to a half-understood outcome.

If the price of more openness is delay, it is worth paying. At one point, Democrats had hoped to wrap up an impeachment vote before Washington dispersed for Thanksgiving at the end of November. Now the end of the year is more feasible. When the vote comes, another partisan split seems inevitable. GOP members of Congress attack the motives of the Democrats (which is not quite the same as protesting the president’s innocence). If their loyalty is replicated in the Senate, Mr Trump will be acquitted.

Until now, Republicans have been able to use the secrecy of the process as proof of its unfairness. Given how much we already know about the president’s behaviour, it was always a weak line. It is about to become an untenable one.

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