Hundreds of thousands of Americans - some Trump supporters, others his foes, and many just patriotic vacationers - were gathering on the sprawling National Mall in the center of Washington for concerts, fireworks and an address by the president that threatened to smash the apolitical tradition of the US national day holiday.
Crowds were growing despite stifling heat in the afternoon outside the highly secured monument to Abraham Lincoln, the 19th century president.
A short cloudburst doused and cooled the spectators, and more storms forecast for the Washington region potentially threatened Trump's speech.
Although the "Salute to America" show is officially open to the public, the section directly in front of Trump is reserved for VIP Republican donors and allies, as well as top military brass.
Those without tickets had to stay outside at least two layers of fencing and will have to watch on giant screens.
Trump's decision to make an address and turn the event into a celebration of the US military has angered his opponents and upset local Washington officials, accustomed to arranging July 4th events without political interference.
"What, I wonder, will Donald Trump say this evening when he speaks to the nation at an event designed more to stroke his ego than celebrate American ideals?" said leading Democratic White House contender Joe Biden in a speech in Iowa.
But some visitors readily approved the idea.
"It's going to be fabulous, I'm excited, said Dee Ranson, a 55-year-old from Florida visiting with her family.
"I think it's awesome, I think it shows courage and patriotic enthusiasm. He is not afraid," she said of Trump.
Trump was scheduled to speak at 6:30 pm (2230 GMT), ahead of a flyover aimed at displaying US air power. However, the address could be delayed by stormy weather.
Excerpts of the speech released ahead of time focused on unity and support for the armed forces.
"Today, we come together as one nation with this very special Salute to America," Trump was expected to say.
"We celebrate our history, our people, and the heroes who proudly defend our flag - the brave men and women of the United States Military!"
Scaled-back display of might
Trump originally wanted a grand military parade for the holiday, ostensibly inspired by France's rollout of its military might on its own national day.
But instead, he has gotten a scaled-back version, with some US armored vehicles parked for display near the monument and a flyover.
That will include the modified Boeing 747 used as the president's own Air Force One, and F-35s fighters and jets from the Navy's Blue Angels air show acrobatics team.
But it is also drawing demonstrators attacking Trump's alleged militarism.
Leftist organization Code Pink is deploying its "Baby Trump" blimp, a large inflated doll depicting the president in diapers.
But the National Park Service denied permission to fill the inflatable with helium, so the cranky, infantile Trump balloon will stay on the ground.
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