President Donald Trump has thrown the US government into crisis by accusing his predecessor, Barack Obama, of wiretapping his phones during the election campaign. That the accusation was made in an early-morning tweet does not make the situation any less serious. Nor does the fact that Mr Trump has tweeted nonsense in the past. His false claim that millions of people voted illegally in November’s election was not a direct and specific allegation against a sitting president. The sources of and evidence for this accusation against Mr Obama must now be established. If they are not, the claims can only have a corrosive effect on trust in the institutions of government, without which the country cannot function.

The response of the president’s spokespeople has made the situation more opaque and therefore more urgent. They argue that the president’s statements are based on stories that have been widely reported. The problem is that the stories in question do not support Mr Trump’s assertions.

The articles say that US law enforcement and intelligence agencies were investigating connections between Russia and the Trump campaign. The New York Times reported, citing an anonymous official source, that the White House was provided with intelligence from that investigation, drawn from wiretaps. The websites Heatstreet and Breitbart said that the FBI or more broadly “the Obama administration” had requested and received warrants under the Foreign Intelligence Surveillance Act (FISA) to investigate members of the Trump campaign.