Although he fell short, O???Rourke outperformed expectations for any Democrat in the deeply conservative state of Texas, falling less than three percentage points behind Cruz, the Republican incumbent.
???This is going to be a positive campaign that seeks to bring out the very best from every single one of us, that seeks to unite a very divided country,??? O???Rourke said in a video announcement posted on Thursday morning. ???We saw the power of this in Texas.???
O???Rourke stated his intention in the video to ???travel this country and listen to those I seek to serve???. He said he will then return to El Paso on 30 March for a campaign kickoff and invited would-be supporters ???to the greatest grassroots campaign this country has ever seen???.
A onetime guitarist for an El Paso punk band called Foss, prior to his Senate bid, O???Rourke had kept a relatively low profile as a three-term congressman with little name recognition.
But as his upstart campaign to defeat Cruz gained steam, O???Rourke transformed into an overnight celebrity who earned the approval of public figures ranging from Beyonc?? to LeBron James. He went on to amass a record-breaking $38m in the third quarter of his race against Cruz, in what will likely mark a major fundraising advantage over the other Democratic contenders.
The president has also mocked O???Rourke???s nickname, calling him ???a stone-cold phony named Robert Francis O???Rourke, sometimes referred to as Beto,??? during a stump speech for Cruz during the midterms.
???He pretends to be a moderate,??? Trump continued, ???but he???s actually a radical open-borders left-winger???. On the left, some have in fact labeled O???Rourke a moderate who has yet to prove his progressive bona fides .
In separate remarks to reporters that same day, Trump also called O???Rourke ???highly overrated???.
???When I heard about him, I figured he must be something a little special. He???s not,??? Trump said.
O???Rourke has the backing of a network of advocates who have independently been campaigning for him to run. The Draft Beto organisation has already mobilised thousands of grassroots volunteers, many on college campuses, and held 30 house parties in nine states.
Its co-founder Nate Lerner told the Guardian: ???Now the real work begins. We???re shifting the grassroots, financial, and political resources we???ve built over to Beto???s campaign and focusing entirely on electing Beto in 2020.???
For all the buzz about his charisma, grassroots appeal and dedication to campaigning for every last vote, O???Rourke does not have a substantial track record in Washington for legislation or making his voice heard on domestic and foreign policy.
Meanwhile, Republican opposition campaigning ahead of O???Rourke???s swing through Iowa this week is focused on portraying him as a product of white wealth and privilege, and is ???no Obama???, who overcame significant struggles to reach the top.