The Democratic governor of Virginia apologized for his appearance in a “racist and offensive” costume in his medical school yearbook, but defied intense and bipartisan calls to step down, saying he intended to serve out his term.

“I am deeply sorry for the decision I made to appear as I did in this photo and for the hurt that decision caused then and now,” Ralph Northam said in a statement.

The Democratic presidential hopefuls Cory Booker, Kamala Harris and Elizabeth Warren led calls for Northam to quit, a gathering chorus which was joined by activist groups and on Saturday by the former vice-president Joe Biden and the Vermont senator Bernie Sanders.

Despite this, on Saturday, the Virginia Democratic party said that though it had “made the decision to let Governor Northam do the correct thing and resign this morning” it had “gotten word he will not do so this morning”.

The photograph shows a person in blackface standing next to a person wearing the white robes and hood of the Ku Klux Klan. It is not apparent which figure is Northam, and the governor’s statement did not clarify the point, stating only it showed “me from my 1984 medical school yearbook in a costume that is clearly racist and offensive”.

Other photographs on the yearbook page show Northam in a suit, wearing a cowboy hat and sitting by a car. The page includes the following quote: “There are more old drunks than old doctors in this world so I think I’ll have another beer.”

Northam, 59, was elected in 2017, having served as lieutenant governor. He also served in the US army, attended Eastern Virginia Medical School and worked as a pediatric neurologist.

Ralph Northam in Richmond, Virginia, on Thursday. Photograph: Steve Helber/AP

His admission that he was one of the figures in the photograph drew immediate calls for his resignation from across the political spectrum.

“Leaders are called to a higher standard, and the stain of racism should have no place in the halls of government,” tweeted the California senator Harris, a leading contender for the Democratic nomination for president. “The governor of Virginia should step aside so the public can heal and move forward together.”

Booker, the New Jersey senator who, like Harris, is African American, tweeted: “These images arouse centuries of anger, anguish, and racist violence and they’ve eroded all confidence in Gov[ernor] Northam’s ability to lead. We should expect more from our elected officials. He should resign.”

Other Democratic presidential hopefuls, senators Elizabeth Warren and Kirsten Gillibrand and the former housing secretary Julián Castro, called on Northam to go, as did leaders of the National Association for the Advancement of Colored People, MoveOn, the pro-choice group Naral and the Priorities USA Super Pac.

The Virginia Legislative Black Caucus (VLBC), state House Democratic Caucus and state Senate Democratic Caucus said Northam should go. Virginia’s two Democratic US senators, Tim Kaine and Mark Warner, condemned the picture but did not immediately call for Northam’s resignation.

A Democratic predecessor as governor, Terry McAuliffe, said “this has been a heartbreaking day” and praised Northam as a friend. But he added: “His actions on display in this photo were racist, unacceptable and inexcusable at any age and any time. The situation that he has put himself and the Commonwealth of Virginia in is untenable. It’s time for Ralph to step down, and for the Commonwealth to move forward.”

The Republican party of Virginia tweeted: “What Ralph Northam did was unforgivable. Given his statements on the right to life coupled with the most recent revelations, he has lost the moral authority to continue to govern and should resign immediately.”

The only black Republican US senator, Tim Scott of South Carolina, wrote: “While a quick apology is good, it does not excuse the choices made by [Northam] as an adult enrolled in medical school.” Scott said he hoped “the people of Virginia will make their voices heard; I hope they will shout far [and] wide that there are consequences for such showcases of prejudice [and] hate”.

He betrayed us State senator Louise Lucas

Black lawmakers said they met Northam on Friday and appreciated his service. “But given what was revealed today, it is clear he can no longer effectively serve as governor,” the VLBC said. “It is time for him to resign, so that Virginia can begin the process of healing.”

Later on Friday, Northam issued a video.

“I cannot change the decisions I made, nor can I undo the harm my behavior caused then and today,” he said. “But I accept responsibility for my past actions, and I am ready to do the hard work of regaining your trust … I am committed to continuing that fight through the remainder of my term.”

The state senator Louise Lucas, an ally of Northam, was unmoved, telling the Associated Press black leaders felt “mortally wounded” and the governor should go.

“He betrayed us,” she said.

Virginia has been grappling with its legacy of slavery and Jim Crow. Approximately 50 members of the Klan rallied in Charlottesville in July 2017 over plans to remove a statue of Confederate general Robert E Lee from a city park. The next month, a gathering of hundreds of white nationalists in the city turned deadly.

Northam was elected that November, a win seen by many as a rebuke of Donald Trump and Republican support of Confederate “heritage”.

Should Northam resign, he would be succeeded by his lieutenant governor, Justin Fairfax, the second African American elected to statewide office in Virginia. Last month, the Washington Post reported on a small act of protest by Fairfax, who declined to participate in a state senate tribute to Lee.

On Saturday, Biden wrote: “There is no place for racism in America. Governor Northam has lost all moral authority and should resign immediately, Justin Fairfax is the leader Virginia needs now.”

Sanders agreed. The lieutenant governor, he said, should “step in and begin a new day for Virginia”.