First the New York Times. Now, The Washington Post finds itself in litigation with the sitting president of the United States, Donald Trump.

The president's re-election campaign filed the Post suit in federal court on Tuesday, basing its allegations on two articles. The campaign alleges both pieces contain false claims – even though both are labelled online as "Opinion" pieces at the top of both pages.

In one, written by Paul Waldman, the campaign takes umbrage with the author's opinion that "who knows what sort of aid" Russia and North Korea might give Mr Trump's re-election bid after he invited their help. The president's campaign contends both Post pieces are defamatory.

Last week, when asked about the Times piece, Mr Trump told a reporter he sued because it went beyond just an opinion.

There are plenty of passages that likely upset the president, but several stand out.

The Times piece, penned by former executive editor Max Frankel, ends by alleging "despite great resistance in Congress, President Trump has watered down the sanctions and otherwise appeased Russian interests, even at the expense of America's allies. Call it the art of the deal."