Donald Trump is set to be investigated by Congress over whether he used “the instruments of state power” to punish media outlets, a senior Democrat has said.

Adam Schiff, who is expected to chair the House intelligence committee next year, said Democrats plan to probe the Trump administration on two fronts following his party’s gains in the midterm elections.

First, they intend to find out if the president attempted to punish Amazon by pressuring the US Post Office to hike shipping prices for the world’s biggest online retailer.

Jeff Bezos, Amazon’s chief executive, owns The Washington Post – a newspaper Mr Trump considers a foe and has regularly branded the “Amazon Washington Post”.

“It is very squarely within our responsibility to find out,” Mr Schiff told Axios in an interview that aired on Sunday on HBO.

He said Mr Trump “was secretly meeting with the postmaster (general) in an effort to browbeat the postmaster into raising postal rates on Amazon … This appears to be an effort by the president to use the instruments of state power to punish Jeff Bezos and The Washington Post,” Mr Schiff said.