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The American stock market took its biggest plunge since September on Wednesday, following reports that President Trump asked former FBI Director James Comey to stop an investigation into former national security adviser Michael Flynn.

The Dow Jones index closed 373 points lower on Wednesday in a sign of low confidence by the market towards the Trump administration.

A growing number of Democrats are calling for the president's impeachment, citing fears that he obstructed justice by interfering in the FBI's investigation.

Meanwhile, misgivings towards the state of the presidency are also growing on the opposite side of the house, with some Republicans also coming out against Trump.

"Watergate took many months and this thing seems to be taking hours," said senior Republican party member, Senator John McCain.

Two senate committees are seeking copies of internal memos, reportedly written by Comey, that describe conversations between Trump and the former head of the FBI.

These memos could help lawmakers understand whether the President did seek to block investigations into Flynn's dealings with Russian officials.

The New York Times reported on Tuesday that Trump told Comey in a February 15 meeting to drop the FBI's investigation into Flynn.

"I hope you can see your way clear to letting this go, to letting Flynn go," Trump reportedly told Comey.

Flynn was forced to resign his position in February after it was revealed that he had lied about his meetings with the Russian Ambassador to the US, Sergey Kislyak.

The Senate intelligence committee has invited Comey to testify but his attendance has not been confirmed.

"I believe the American people will get a chance to hear from Director Comey shortly," said Senator Mark Warner (Democrat).