By Eric Bradner

CNN

WASHINGTON (CNN) -- The mother of Matthew Shepard is urging senators to oppose Alabama Sen. Jeff Sessions' nomination to become President-elect Donald Trump's attorney general, citing his opposition to hate crimes legislation.

In a report issued by the Human Rights Campaign, Judy Shepard -- whose son Matthew was beaten and left to die in Wyoming in 1998 in a crime motivated by anti-gay sentiment -- blasted Sessions for opposing the Hate Crimes Prevention Act, which President Barack Obama signed into law in 2009.

A Sessions spokeswoman, meanwhile, says he would enforce hate crimes laws as attorney general -- even though he opposed them in the Senate over constitutional concerns.

Judy Shepard's comments come just before Sessions' confirmation hearing, set to begin at 9:30 a.m. ET Tuesday before the Senate Judiciary Committee.

Judy Shepard wrote that Democrats and Republicans had helped push for hate crimes legislation in Congress, but that "Senator Jeff Sessions was not one of these members."

"In fact, Senator Sessions strongly opposed the hate crimes bill -- characterizing hate crimes as mere 'thought crimes.' Unfortunately, Senator Sessions believes that hate crimes are, what he describes as, mere 'thought crimes,'" she wrote.

"My son was not killed by 'thoughts' or because his murderers said hateful things. My son was brutally beaten ... with the butt of a .357 magnum pistol, tied him to a fence, and left him to die in freezing temperatures because he was gay. Senator Sessions' repeated efforts to diminish the life-changing acts of violence covered by the Hate Crimes Prevention Act horrified me then, as a parent who knows the true cost of hate, and it terrifies me today to see that this same person is now being nominated as the country's highest authority to represent justice and equal protection under the law for all Americans."

She wrote that Sessions "has forfeited opportunity after opportunity to stand up for people like my son Matt and has, instead, used his position of power to target them for increased discrimination and marginalization, thus encouraging violence and other acts deemed to be hate crimes."

The report also cites other positions Sessions has taken that the Human Rights Campaign argues are anti-LGBT -- including his opposition to the repeal of the military's "don't ask, don't tell" policy, his support for anti-sodomy laws and his opposition to same-sex marriage.

The report comes as Sessions is under fire for his stances on LGBT rights.

The pro-LGBT rights Lambda Legal CEO Rachel Tiven said Monday that Sessions "has demeaned and dismissed LGBT people at every turn -- especially those of us who are also immigrants, women, and people of color. He is a lifelong opponent of civil rights, and he is unfit to serve as Attorney General."

A source familiar with Sessions' thinking said he opposed hate crimes legislation in the Senate over concerns about its constitutionality, including vague terms that could have allowed re-prosecution of those who had been acquitted and the law's lack of a direct tie to Congress' power over interstate commerce.

A Sessions spokeswoman said as attorney general, he would enforce hate crimes laws -- even though he did not back them as a senator.

"Senator Sessions believes that all Americans, no matter their background, deserve effective protection from violence and that crimes committed on the basis of prejudice are unquestionably repugnant," said Sarah Isgur Flores, a Sessions spokeswoman. "While he may have had disagreements about what was the most effective policy to combat such crimes, as Attorney General, he will be fully committed to enforcing the laws -- even those for which he did not vote."