“This falls into that category of it’s never too late to do the right thing,” said Representative Ron Kind, Democrat of Wisconsin, who co-sponsored legislation that allowed Lieutenant Cushing to receive the medal so long after the war. “Many military historians for years scratched their heads wondering how he was overlooked for the Medal of Honor at the time. Coming around full circle 150 years after the fact is the right thing to do.”

Nearly half of the previous 3,490 recipients of the Medal of Honor fought in the Civil War, but none waited as long as Lieutenant Cushing. The approval of the medal by the first black president adds a certain historical coda to the saga, underscoring how much has changed in this country since Lieutenant Cushing gave his life in a war that ended slavery in the United States.

The long delay owes to a variety of factors that speak to how Washington works, or does not. At the time of Lieutenant Cushing’s death, the medal was not awarded posthumously, so he was ineligible. Once the rules changed and his cause was taken up, it lingered for years in the bureaucratic and legislative trenches of the capital, where some worried that honoring him would open the floodgates to other requests. Mr. Kind said some Southern colleagues were also less than enthusiastic.

“There was some resistance to awarding a Union soldier the congressional medal at Gettysburg even 150 years after the fact,” Mr. Kind said. “They didn’t want us refighting the Civil War all over again. It’s still sensitive. But we were finally able to bridge that gulf and get it done.”

The nation’s leadership has lately been revisiting veterans of other conflicts who were not decorated. This past spring, Mr. Obama awarded the Medal of Honor to 24 veterans from World War II, Korea and Vietnam who had been overlooked, most because of their racial or ethnic backgrounds. The White House announced Tuesday that Mr. Obama would likewise honor two more Vietnam veterans on Sept. 15.