"Let us continue to fearlessly champion the issues and causes that brought us together," O'Rourke wrote. "Whether it is ending the epidemic of gun violence or dismantling structural racism or successfully confronting climate change before it is too late, we will continue to organize and mobilize and act in the best interests of America."
The 47-year-old also expressed his support for whichever candidate becomes the Democratic nominee, saying "we will be well served by any one of them."
Speaking to supporters at rally in Mississippi late Friday, US President Donald Trump took a jab at O'Rourke, calling him a "poor bastard" who quit the presidential race "like a dog."
"See, people think this is easy but it's not so easy," Trump went on, saying the decision was "pathetic" before also criticizing the Texan's calls for stricter gun laws.
O'Rourke launched his presidential bid in March after narrowly failing to oust Texas Republican Senator Ted Cruz in 2018.
In the Senate race, he drew comparisons to Barack Obama for his youth and charismatic oratory style and did better than any Democrat had in Texas for decades.
But O'Rourke failed to build significant support in the presidential race crowded with more than 20 Democratic candidates.
Former vice president Joe Biden and senators Elizabeth Warren and Bernie Sanders are currently the favourites win the Democratic 2020 presidential nomination.
Trump also took the opportunity of Friday's rally to insult Biden, referring to him as "Sleepy Joe."
"I'm afraid if he gets the nomination he'll be so slow we'll have the lowest-rated debates in history," Trump said.
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