Protesters greeted the president with cries of "Shame!"as he arrived at the $35,000 (£27,000) per person bash.
Many were unhappy with the Republican healthcare plan, holding placards that said "Healthcare, not tax cuts".
Holding the fundraising event at Trump International Hotel has increased concerns about conflicts of interest.
Richard Painter, who served as the chief ethics lawyer in the White House for former President George W Bush, said it was unacceptable for the president to be potentially benefiting financially from this kind of event. He should have picked another hotel, he said.
But Kathleen Clark, a law professor who specialises in government ethics at Washington University in St. Louis, told USA Today it did not break any laws.
It is not clear if the hotel is being paid to host the event.
Republican National Committee officials have told the media they expect the evening to raise about $10m, with about 300 places available.
Not all the money raised will go towards the Trump 2020 campaign - some will go to other Republican Party causes.
It is unusual for a president to raise cash for re-election so early in his first term, only five months since the former property developer took office.
"Of course he is running for re-election," White House spokeswoman Sarah Huckabee Sanders told reporters on Wednesday.
Reporters have been barred from attending the event.
The president has previously been criticised for entertaining foreign leaders at another of his properties in Florida.
A lawsuit filed in June argued President Trump was "flagrantly violating the constitution" by accepting payments from foreign governments, a charge the White House has strongly denied.
Photo: AFP (Healthcare was the main subject of protests)