'The question is no longer whether there are grounds to impeach Trump. The practical question is whether there is the political will': Getty Images

There are now four grounds to impeach Donald Trump and a fifth is "on its way", according to former Labour Secretary Robert Reich.

Posting on Twitter, Mr Reich outlined the four reasons he thinks Mr Trump could be impeached.

He said Mr Trump is "'unfaithfully' executing his duties" by accusing former President Barack Obama of "undertaking an illegal (and impeachable) act."

Last weekend, Mr Trump accused Mr Obama of wiretapping his phones in Trump Tower, though he provided no evidence for his claim.

A spokesman for Mr Obama denied he ever ordered the wiretapping of any US citizen.

Mr Reich also said although part of the constitution forbids government officials from taking things of value from foreign governments, "Trump is making big money off his Trump International Hotel by steering foreign diplomatic delegations to it, and will make a bundle off China's recent decision to grant his trademark applications for the Trump brand — decisions Chinese authorities arrived at directly because of decisions Trump has made as president."

China recently granted preliminary approval for dozens of Trump-branded businesses, including new hotels, spas, massage parlours and personal security services.

The former Labor Secretary also said Mr Trump's ban on travel from six Muslim-majority countries violates the 1st Amendment of the Constitution, which bans any law "respecting an establishment of religion or prohibiting the free exercise thereof."

He also said Mr Trump "labelling the press the 'enemy of the people' and choosing whom he invites to news conferences based on whether they've given him favourable coverage" could be another reason for impeachment, as he said it violates the 1st Amendment on the freedom of the press.

Finally, he wrote: "Article III Section 3 of the Constitution defines 'treason against the United States' as 'adhering to their enemies, giving them aid and comfort.' Evidence is mounting that Trump colluded with Russian operatives to win the 2016 presidential election."

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Mr Trump has repeatedly denied his team had contact with Russian officials during the 2016 election, but was revealed to have met with Russia's US ambassador at the height of his campaign.

"The question is no longer whether there are grounds to impeach Trump. The practical question is whether there is the political will," Mr Reich concluded.

"As long as Republicans remain in the majority in the House (where a bill of Impeachment originates), it's unlikely.

"Another reason why it's critically important to flip the House in 2018."