Pausing, Bush quipped in a low voice: "I know how he felt."
Bush was cheered and applauded by his supporters, who rose to their feet five times during the 15-minute ceremony.
When a red drape was removed from his portrait, the president cocked his head to view the finished work with his hands on his hips. He shrugged.
The 43d president called the work by Alabama artist Mark Carder "a really fine job with a challenging subject."
Bush is the 23d president included in the club's renowned presidential portrait collection, which includes one Democrat (Andrew Jackson, the seventh president) and all but two Republicans (Benjamin Harrison, the 23d president, and Warren G. Harding, the 29th).
"I'm keeping good company," Bush said.
The portrait, underwritten by Union League members Bruce and Eileen Hooper, shows Bush in the Treaty Room of the White House.
Bush is the first president since Herbert Hoover to have his portrait unveiled at the Union League while in office.
In his remarks, a relaxed and playful Bush acknowledged the presence of Republican Sen. Arlen Specter in the front row of the hall.
"It's been a joy to work with you," Bush told Pennsylvania's senior senator, "most of the time."
Of his mother, Barbara, who was recently released from a hospital, Bush told the audience that she is "a tough old bird" who was recovering.
In a more serious moment, Bush said that when someone asked him what it was like to be president, he responded, "Some days you're happy. Some days you're not so happy. And every day you're joyous to serve the United States."
Bush also received the Union League's Gold Medal, one of only 37 recipients since 1863. Others include Vice President Cheney and former Bush cabinet members Donald Rumsfeld and Tom Ridge.
