By Daniel Trotta

NEW YORK (Reuters) - Transgender advocates vowed on Wednesday to resist any effort to reverse or slow down an expansion of transgender rights made under President Barack Obama, fearing a rollback of his directives once Donald Trump succeeds him in the White House.

During his eight years in the Oval Office, Obama championed lesbian, gay, bisexual and transgender (LGBT) rights, and Democrat Hillary Clinton had promised to keep fighting for them if she was elected.

Same-sex marriage became legal in 2015, shifting the LGBT discussion to the still-unfinished agenda of transgender rights. Obama has issued regulations prohibiting health insurers from denying coverage based on gender identity and directed public schools to allow transgender students to use the public bathrooms of their choice, among other measures.

"We expect that those will disappear," said Rachel B. Tiven, chief executive of Lambda Legal, a leading gay rights group. "Lambda Legal will hold the line when our rights are under attack. We know how to fight the government in the courts."

Trump sent mixed messages on transgender rights during the campaign. But he promised to nominate a conservative justice to fill the vacancy on the Supreme Court and could name an attorney general who may enforce hard-line positions Trump outlined during the campaign.

"We're going to fight them on anything they try to undo," said Mara Keisling, executive director of the National Center for Transgender Equality. "I'm really worried today for trans Muslims and trans immigrants and trans people with disabilities who have been threatened and denigrated by Mr. Trump. Hopefully, that was all part of some sick performance."

With the U.S. Supreme Court split between four liberals and four conservatives, Trump could appoint a conservative to tip the balance, possibly in time to decide a case involving a transgender boy who has sued to use the male restroom at his public school in Virginia.

Shannon Minter, legal director for the National Center for Lesbian Rights, called Trump's victory "a huge wake-up call" that placed an onus on transgender people to "explain who we are" to Trump voters who may be intolerant of or ignorant about transgender people.

"In a very real sense, our lives, our ability to work, to attend school, to obtain health care, are at the mercy of a new administration," Minter said.

Trump's Justice Department and appointees to the federal bench could also have an impact on two provisions of federal law banning discrimination on the basis of sex - Title VII of the Civil Rights Act, which covers employment, and Title IX of the Education Amendments of 1972.

The Obama administration has taken a firm stance that the reference to "sex" protects transgender people.

The courts have generally agreed, and Jill Gaulding, co-founder and legal director of Gender Justice, said that trend was likely to continue regardless of who is president.

"We can never take any rights for granted but I also think it's important to remember how much progress we have made in recent years and decades," she said. "It's not easy to repeal that kind of progress."

(Editing by Frank McGurty and Dan Grebler)