President Donald Trump's young Democratic rival has called his State of the Union address "unsettling", refusing to apologise for defiantly remaining seated during a standing ovation.

Alexandria Ocasio-Cortez is a political newcomer, elected to the US House of Representatives in the 2018 midterms.

Serving the southern area of the Bronx and Queens in New York City, at age 29 Ocasio-Cortez is the youngest woman ever elected to Congress.

Ocasio-Cortez, centre, listens as President Donald Trump delivers his State of the Union address. Photo / AP
Ocasio-Cortez, centre, listens as President Donald Trump delivers his State of the Union address. Photo / AP

She sits on the left of politics, describing herself as a democratic socialist, and is often criticised for being "teenage" in her views, claims she fights back against ferociously.

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She has become well known for engaging with other politicians and celebrities who name her on Twitter.

Female leaders dressed en masse in white to honour the female suffragette movement, which led to the 19th amendment and women winning the right to vote.

During his address, Trump praised women's increasing role in the workforce, and the number of women serving in Congress. There are currently 102 women serving in the US House of Representatives.

"No one has benefited more from our thriving economy than women, who have filled 58 per cent of the new jobs created in the last year," the President said.

"All Americans can be proud that we have more women in the workforce than ever before — and exactly one century after the Congress passed the Constitutional amendment giving women the right to vote, we also have more women serving in the Congress than ever before."

Rep. Alexandria Ocasio-Cortez (D-NY) watches President Donald Trump's State of the Union address in the chamber of the U.S. House of Representatives. Photo / Getty Images
Rep. Alexandria Ocasio-Cortez (D-NY) watches President Donald Trump's State of the Union address in the chamber of the U.S. House of Representatives. Photo / Getty Images
Trump delivers his State of the Union address to a joint session of Congress on Capitol Hill in Washington. Photo / AP
Trump delivers his State of the Union address to a joint session of Congress on Capitol Hill in Washington. Photo / AP

Following these statements, different Democratic women stood up to applaud and chant "USA! USA! USA!", led by Nancy Pelosi — leader of the recent government shutdown.

Ocasio-Cortez was not among this contingent, and remained seated throughout the entirety of Mr Trump's address.

Mr Trump also went on to say the government was launching a "Government-wide initiative focused on economic empowerment for women in developing countries".

Ocasio-Cortez remained steely faced throughout the address, refusing to get up from her seat as her colleagues gave the President a standing ovation. Critics said she was having a "bad night".

"(Ocasio-Cortez) had a rare bad night, looking not spirited, warm and original as usual but sullen, teenage and at a loss," said Peggy Noonan, a columnist from the Wall Street Journal via Twitter.

"Why should I be 'spirited and warm' for this embarrassment of a #SOTU? (State of the Union?)" she tweeted back.

"Tonight was an unsettling night for our country. The president failed to offer any plan, any vision at all, for our future.

"We're flying without a pilot. And I'm not here to comfort anyone about that fact."

Women members of Congress cheer after President Donald Trump acknowledges more women in Congress during his State of the Union address. Photo / AP
Women members of Congress cheer after President Donald Trump acknowledges more women in Congress during his State of the Union address. Photo / AP

"I think he's scared. He sees that everything is closing in on him," she said.

"I think he sees himself losing on the issues, he sees himself losing on the wall in the southern border, and he needs to grasp at an attack and this is his way of doing it."

"They can get used to it because I won't be smiling for this president anytime soon," she told the New York Post.

Democratic members of Congress pose for a photo before President Donald Trump delivers his State of the Union address. Photo / AP
Democratic members of Congress pose for a photo before President Donald Trump delivers his State of the Union address. Photo / AP

"I think the President was unprepared. I don't think he did his homework," she told MSNBC after the address.

"He feels himself losing on the issues. Every single policy proposal that we have adopted and presented to the public has been overwhelmingly popular.

"This is an issue of an authoritarian regime versus democracy."