El Paso, Texas (CNN) Beto O'Rourke challenged President Donald Trump in his most direct and public way yet Monday, lambasting Trump's calls for a border wall on a stage a little more than a football field away from Trump's rally here.

"With the eyes of the country upon us, all of us together are going to make our stand, here in one of the safest cities in the United States of America -- safe not because of walls, but in spite of walls," the former Texas congressman told a sprawling crowd of thousands that had filled an outdoor baseball field after marching a mile. "We treat each other with dignity and respect. That is the way we make our community and our country safe."

The subtext of the day was O'Rourke's potential run for the Democratic 2020 presidential nomination and the opportunity to take on Trump in a general election. As he weighs entering an already crowded field, Monday was a test of O'Rourke's ability to confront the President with a national audience -- including Democratic primary voters who polls have shown are particularly concerned about electability in choosing their nominee.

O'Rourke's event was so close to Trump's that "God Bless America" could be heard blaring over the nearby loudspeakers. Trump took the stage just as O'Rourke wrapped up, and his speech was audible as O'Rourke's supporters left the ball field.

The former Texas congressman was on clearly on Trump's mind. At his rally, the President called O'Rourke "a young man who has got very little going for himself except he's got a great first name."

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