WASHINGTON — It was a lavish birthday party for Donald Trump Jr.’s girlfriend, Kimberly Guilfoyle. The setting was Mar-a-Lago, President Trump’s private club in Palm Beach, Fla. The guest list included dozens of Trump family members and friends.

But when it came to picking up the tab, hands went out to other attendees. Among them were at least four whose families are financial supporters of the president’s re-election campaign, for which Ms. Guilfoyle helps lead the fund-raising. They ended up pitching in tens of thousands of dollars, passed along to Mar-a-Lago, to help pay for what two people familiar with the planning said was a $50,000 celebration of Ms. Guilfoyle’s 51st birthday.

The hourslong bash on the evening of March 7 has since emerged as a snapshot of the cavalier way Mr. Trump and his team initially handled the coronavirus outbreak. At least one attendee — a Brazilian government official who stopped by the party briefly — has tested positive for the virus, while another — Representative Matt Gaetz of Florida — self-quarantined, though he later announced he had tested negative for the virus.

But the funding provided for the party by Mr. Trump’s supporters, which has not been previously reported, also underscored other characteristics that have defined his political career: His reliance on other people’s money rather than his own fortune, and the overlap between the president’s private and public roles.