Democrat Beto O'Rourke has announced he is dropping out of the crowded race for the 2020 US presidential nomination, after months of stagnant polling numbers and dwindling finances.

"Though it is difficult to accept, it is clear to me now that this campaign does not have the means to move forward successfully," the former congressman said in a statement.

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"My service to the country will not be as a candidate or as the nominee."

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The charismatic 47-year-old Texan entered the contest with a considerable buzz after narrowly losing a US Senate bid to Republican incumbent Ted Cruz in 2018, a race that saw him raise staggering amounts of campaign donations.

When he launched his presidential bid in March, Mr O'Rourke was viewed as one of the 2020 frontrunners, but on the national stage, he failed to re-ignite the enthusiasm that had propelled him to rockstar status in his Senate race in Texas.

We will work to ensure that the Democratic nominee is successful in defeating Donald Trump in 2020. I can tell you firsthand from having the chance to know the candidates, we will be well served by any one of them, and I’m going to be proud to support whoever she or he is. — Beto O'Rourke (@BetoORourke) November 1, 2019

Polling at about two per cent in recent months - far behind frontrunners Joe Biden, Elizabeth Warren and Bernie Sanders - his performance rarely moved the needle.

Mr O'Rourke's campaign team ruled out another Senate run, at least for now.

"Beto will not be a candidate for US Senate in Texas in 2020," communications director Rob Friedlander told AFP.

Mr O'Rourke has been a fierce critic of President Donald Trump, openly branding him a "racist" who is motivated by white supremacist views.

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Minutes after Mr O'Rourke's announcement, the president mocked his critic and political adversary in a tweet.

"Oh no, Beto just dropped out of race for President despite him saying he was 'born for this.' I don't think so!" Mr Trump said.

Oh no, Beto just dropped out of race for President despite him saying he was “born for this.” I don’t think so! — Donald J. Trump (@realDonaldTrump) November 1, 2019

Mr O'Rourke had expressed particular anger towards Mr Trump after a horrific shooting in his Texas hometown of El Paso in early August, saying the massacre was a "consequence" of Mr Trump's hostile rhetoric towards immigrants.

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One of Mr O'Rourke's signature policy issues was curbing gun violence, and after the tragedy in El Paso, where a gunman slaughtered 22 people using a military-style assault weapon, Mr O'Rourke doubled down on his call for a mandatory gun buyback program.

"Hell yes, we're going to take your AR-15, your AK-47," he said during a Democratic debate in September in one of the more passionate and memorable moments of his campaign.