Donald Trump has branded Beto O'Rourke a "poor bastard" and a "pathetic guy" after the former Texas congressman ended his presidential campaign.

Key points: Beto O'Rourke's campaign fundraising and polling dwindled significantly in recent months

Beto O'Rourke's campaign fundraising and polling dwindled significantly in recent months He struggled to deliver consistent vision and messaging as a presidential candidate

He struggled to deliver consistent vision and messaging as a presidential candidate His decision comes as the Democratic primary campaign approaches the Iowa caucus

The US President's comments at a rally in Tupelo, Mississippi came after Mr O'Rourke confirmed he was pulling out of a crowded Democratic field for next year's election, saying: "My service to the country will not be as a candidate or as the nominee".

Mr Trump told supporters Mr O'Rourke "wasn't smart" and "quit like a dog".

"Oh did you hear? Beto, Beto. Oh that poor bastard. Poor pathetic guy — who was pathetic. Remember the arms flailing?" he said.

"He came out of Texas, a very hot political property, and he went back as cold as you can be… He was against religion. He was against you having a gun. And he was against oil.

"Beto was nasty and he said that he was born for it, like he was born from heaven. He came down. And if that's the case, some really bad things happened — because he made a total fool out of himself."

Mr O'Rourke had been urged to run for president by many Democrats, including supporters of former president Barack Obama, who were energised by his narrow Senate loss last year in Texas, a reliably Republican state.

An interview in which Mr O'Rourke appeared to say he was "born" to be in presidential politics spurred claims he had a sense of entitlement. ( Reuters: Mike Segar )

He raised money from small donors across the country, visited every county in Texas and used social media and livestreaming video to engage directly with voters, but ultimately lost to incumbent Republican senator Ted Cruz by 3 percentage points.

Mr O'Rourke struggled to replicate his campaign model in the presidential primary, and both his polling and his fundraising dwindled significantly in recent months.

"Though it is difficult to accept, it is clear to me now that this campaign does not have the means to move forward successfully," he wrote on Medium.

"Acknowledging this now is in the best interests of those in the campaign; it is in the best interests of this party as we seek to unify around a nominee; and it is in the best interests of the country."

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Democratic primary entering critical stretch

Mr O'Rourke's decision came as the Democratic primary entered a critical stretch.

With three months until the start of Iowa caucuses, polls consistently show a trio of candidates leading the way: former vice-president Joe Biden, Massachusetts senator Elizabeth Warren and Vermont senator Bernie Sanders, with Pete Buttigieg, the mayor of South Bend, Indiana, showing strength in Iowa as well.

Lower polling candidates face difficult questions about whether they have the money to sustain a campaign through the first primary contests.

Earlier this week Kamala Harris, another candidate who entered the race to much fanfare, announced she was downscaling her campaign, laying off some staffers and reorienting almost exclusively to focus on Iowa.

Mr O'Rourke entered the race as the feelgood, dynamic candidate who had the ability to appeal to both Republicans and Democrats and work across the aisle in Washington.

But he immediately faced criticism that he had a sense of entitlement, particularly after the release of a Vanity Fair interview on the eve of his campaign launch in which he appeared to say he was "born" to be in presidential politics.

Mr O'Rourke did not endorse another Democrat for the nomination, saying the country would be well served by any of the other candidates. ( AP: Charlie Neibergall )

He also struggled with consistent vision and messaging as a presidential candidate.

He spent several weeks trying to build his campaign around climate change, calling global warming the greatest existential threat the country had ever faced.

After a gunman opened fire at a Walmart in his hometown of El Paso, killing 22 people, O'Rourke more heavily embraced gun control, saying he would take assault weapons away from existing owners.

Mr O'Rourke did not endorse another Democrat for the nomination, saying the country would be well served by any of the other candidates, "and I'm going to be proud to support whoever that nominee is".