The US president tweeted "WE WILL NEVER FORGET" alongside a video showing footage of the 11 September 2001 terrorist attacks spliced with a speech by Representative Ilhan Omar.

"Some people did something," she is seen saying, in between footage of planes hitting the Twin Towers and people fleeing the buildings.

Republicans have accused her of downplaying the attacks, but Democrats have largely rallied to her defence, saying she had been quoted out of context and some accusing Mr Trump of inciting violence against her and Muslims. Here is how the row developed.

Ms Omar won a Minnesota seat in the House of Representatives last November, becoming one of the first two Muslim women ever elected to the US Congress.

Her family originally came to the US as refugees from Somalia and she is the first congresswoman to wear the hijab.

Despite being a newcomer to Washington, this is not the first time Ms Omar has made headlines.

She has been accused of anti-Semitism over comments she made about Israel and pro-Israel lobbyists. After being rebuked last month, including by Democrats, she apologised and said she was "listening and learning".

The congresswoman has also raised the alarm about anti-Muslim rhetoric surrounding her, in response to a Republican poster that showed her alongside the Twin Towers.

Rightwing politicians were quick to say Omar was dismissing the gravity of 9/11. Dan Crenshaw, a congressman from Texas who as a Navy Seal was seriously injured in Afghanistan, falsely claimed Omar “does not consider [September 11] a terrorist attack on the USA by terrorists”.

Two of the most progressive candidates for the Democratic nomination called on all lawmakers to condemn such attacks. Senator Bernie Sanders called attacks on Omar “disgusting and dangerous” and said Omar would not “back down to Trump’s racism and hate, and neither will we”.

Senator Elizabeth Warren said: “The president is inciting violence against a sitting congresswoman – and an entire group of Americans based on their religion. It’s disgusting. It’s shameful. And any elected leader who refuses to condemn it shares responsibility for it.”

Pete Buttigieg, the mayor of South Bend, Indiana who has surged in 2020 polls, wrote: “After 9/11 we all said we were changed. That we were stronger and more united. That’s what ‘never forget’ was about. Now, a president uses that dark day to incite his base against a member of Congress, as if for sport. As if we learned nothing that day about the workings of hate.”

Buttigieg added: “The threats against the life of [Omar] make clear what is at stake.”

Among other candidates, former Hud secretary Julián Castro said he stood with Omar and “others targeted by the president’s anti-Muslim rhetoric”. Former congressman Beto O’Rourke said: “We are stronger than this president’s hatred and Islamophobia. Do not let him drive us apart or make us afraid.”

The California senator Kamala Harris wrote: “For two years, this president has used the most powerful platform in the world to sow hate [and] division. Putting the safety of a sitting member of Congress at risk [and] vilifying a whole religion is beyond the pale. I’ll be blunt – we must defeat him.”