Democrat Beto O'Rourke has announced he is running for president, joining a crowded field of candidates vying to challenge Donald Trump in 2020.

"The only way for us to live up to the promise of America is to give it our all and to give it for all of us," the 46-year-old said in a video.

He has been discussed as a potential candidate ever since his unexpectedly tight race last year to unseat Senator Ted Cruz, when his grassroots campaign of inclusion and getting people politically involved gained national attention.

Mr O'Rourke now enters a big pool of Democrats seeking to oust Mr Trump.

They include several US senators: Elizabeth Warren, Kamala Harris, Cory Booker, Kirsten Gillibrand, Amy Klobuchar, and the leader among the current candidates, liberal powerhouse Bernie Sanders.

Now, the last main piece of the 2020 election puzzle will be former vice president Joe Biden, who has said he will reveal his political plans soon. 

Mr O'Rourke, a former punk musician who sometimes went skateboarding to blow off steam on the Texas campaign trail, has been tipped to quickly achieve rockstar status with a run.

But that will come with intensifying scrutiny from the media, Democratic power brokers and donors, as well as voters.