The trouble started months ago for Savannah Cunningham, who had long dreamed of becoming a Marine, when she was deluged with lewd messages online from men and learned that an all-male group of Marines was circulating a nude video of her on Facebook.

New waves of requests and obscene comments about her appearance arrived every time the video, initially obtained from a former boyfriend, and other photos taken from her Instagram account were reposted along with her identity. “It was horrible,” Ms. Cunningham, 19, said from her home in Phoenix.

“It was such a creepy invasion of privacy,” she added. “They were actively seeking nude images of me, anything they could get their hands on.”

Given such a raw view of the worst of Marine culture, many women might have been turned off by the military. But Ms. Cunningham was determined to enlist. She ships off to basic training the first week of April.

“Someone needs to stand up and say this does not represent the values of the Marine Corps,” she said. “If not me, then who? Yes, for a long time it was a boys’ club, but there needs to be progress.”

News of the invitation-only Facebook group behind the harassment, a 30,000-member collection of active-duty Marines and veterans called Marines United, has turned up the heat on the long-simmering problem of how women are treated in the Marine Corps.

The Corps leadership, which will face questioning on Tuesday from the Senate Armed Services Committee, has scrambled to assure Marines and the public that the behavior of the Facebook group does not represent the institution. They have vowed to punish anyone who violates regulations.

But the group’s actions in recent days show that it may not be easy to deter. When the original group page was shut down after the news site Reveal uncovered its actions last week, a core group of photo sharers moved to another secret group called MU 2.0. Then, when that was shut down, they moved to one called MU 3.0.

And even as the top enlisted Marine, Sgt. Maj. Ronald L. Green, condemned the group in testimony before Congress last week, members were taunting him online, according to a veteran, James LaPorta, who has been tracking the group’s activities.

“There seems to be no regret,” Mr. LaPorta said. “They were using racial slurs and talking about getting pictures of his wife.”

The group continues to post on anonymous pornography sites. A recent review of images from these sites shows dozens of identifiable women, naked or partly undressed, along with photos of them in uniform.

Thomas Brennan, a Marine veteran and journalist who first reported on the group, said that he had given the names of 55 Marines involved in the photo sharing — including officers ranked as high as lieutenant colonel — to investigators six weeks ago, but that there was no sign that any of them had been removed from duty.

A Marine spokesman said he could not comment on the continuing investigation.

The scandal has now spread to the Army and Navy, which are investigating similar photo sharing groups. But for the tens of thousands of women serving in the military, even successful prosecutions may have little effect on the minefield of bias they say they confront.

More than any other branch of the military, the Marine Corps has resisted integrating women. It still trains recruits separately and fails to give women properly fitting body armor, which the Army has provided for years.

“Almost every woman I know in the Marines has faced this kind of harassment, and you try to show you are tough enough to ignore it,” said Justine Elena, a former Marine captain who served in Afghanistan and now works for “The Daily Show.” “But at some point, by ignoring it, you just condone it.”

She recalled an instance when fellow Marines took photos of women in the bathrooms on a ship a friend was serving on.

“We need to stand up and say that is not what the Marines is about,” she said.

Last week, after hearing of the widespread photo sharing, Ms. Elena set up a fund-raiser called Female Marines United, in which people could show their opposition to Marines United by donating money to support mental health care for Marine veterans.

Ms. Cunningham, who is just a few weeks away from placing her feet on the painted yellow footprints at basic training that are the symbolic first step in becoming a Marine, said that while she had often been torn about how to respond to the online harassment, she had decided that the best response would be to become the best Marine possible.

Always athletic growing up, she made up her mind in high school to join the military, and chose the Marines, she said, because it was the most selective and demanding. She plans to work on a crew loading missiles on Cobra helicopters.

Two years ago, she began working out intensely to prepare — hitting the gym until her arms, which could not do a single pull-up at first, could knock out 14 in a row.

“I wanted to make sure I could do anything male Marines could,” she said. “I didn’t want anyone to hold me to a lower standard.”

At the time, she was dating a Marine, and when he was stationed outside of Arizona, she sent him a short strip tease video.

“I don’t typically do that stuff,” she said. “But for the person you love, you do things to keep a relationship alive.”

The video was soon added to the cache of hundreds of photos and videos of active-duty Marines and veterans — filed with the subject’s name, rank and place of duty — that is being circulated by Marines United and other groups. Ms. Cunningham was notified by male Marines she knew who were members of the group, and she was eventually given access to view the whole collection.

She pored through the files, searching for women she knew so she could alert them, knowing that their colleagues and commanders could see them.

But while she was horrified by the actions of the group, she said she never equated it with the Marine Corps. She now dates a Marine sergeant and said that most of the male Marines she knew were just as disgusted by the photo sharing as she was. That was what made her stick with her plan to enlist.

“We have to be positive examples of the change we want to see,” she said. “Courage, integrity, honor: I want to live those values.”