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Social conservatives opposed to a hate crimes bill that would extend federal protection to gay and transgender victims are mounting a last-ditch effort to defeat it -- even as it nears passage in Congress

The House voted Thursday to make it a federal crime to assault people because of their sexual orientation, significantly expanding the U.S. hate crimes law enacted in the days after the 1968 assassination of civil rights leader Martin Luther King Jr..

The Senate is expected to pass the bill, allowing federal prosecutors for the first time to intervene in cases of violence perpetrated against gays.

No one is arguing against the prosecution of assaults. But opponents, fearing threats to free speech under broad interpretations of the legislation, are pushing voters to contact their senators to voice their displeasure over the expansion of the existing law. They acknowledge the odds are against them.

"It's going to be very difficult to defeat at this stage," said Mathew Staver, founder of Liberty Counsel, a law firm that works on religious freedom cases.

Staver said he is hoping for a "groundswell of support" to stop the bill dead in its tracks and at the very least raise awareness of the bill's far-reaching impact. If that fails, Staver said his groups is "strongly considering" filing a lawsuit based on the broad language of the bill that he says would allow federal intervention into past cases, including ones of alleged rape.

Although it's been 11 years since the gay college student Matthew Shepard, whose name was attached to the legislation, was murdered in the state of Wyoming, his death is still a fresh memory for supporters of the legislation.

But social conservatives, including former President George W. Bush, argue their right to free speech will be jeopardized if it becomes law. They say it could be used to prosecute religious groups who say homosexuality is wrong.

Some have taken that argument to the extreme, saying the bill will lead to the legalization of necrophilia, pedophilia, and bestiality.

"If you're oriented toward animals, bestiality, then that's not something that can be used, held against you or any bias be held against you for that," Rep. Louis Gohmert, R-Texas, said on the House floor Tuesday.

"Which means you'd have to strike any laws against bestiality, if you're oriented toward corpses, toward children…there are all kinds of perversions … but the trouble is, we made amendments to eliminate pedophiles from being included in the definition."

After voting against the bill Gohmert toned the rhetoric down but maintained his opposition.

"There is nothing accomplished by this hate crimes legislation that is not already accomplished by current law in the country," he said in a written statement.

Civil rights groups and their Democratic allies have been trying for more than a decade to broaden the reach of hate crimes law. This time it appears they will succeed. The measure is attached to a must-pass $680 billion defense policy bill and President Obama -- unlike Bush -- is a strong supporter. The House passed the defense bill 281-146, with 15 Democrats and 131 Republicans in opposition.

Many Republicans, normally stalwart supporters of defense bills, voted against it because of the addition of what they referred to as "thought crimes" legislation.

"This is radical social policy that is being put on the defense authorization bill, on the backs of our soldiers, because they probably can't pass it on its own," House Republican leader John Boehner said.

Republican opponents were not assuaged by late changes in the bill to strengthen protections for religious speech and association -- critics argued that pastors expressing beliefs about homosexuality could be prosecuted if their sermons were connected to later acts of violence against gays.

Supporters countered that prosecutions could occur only when bodily injury is involved, and no minister or protester could be targeted for expressing opposition to homosexuality.

The bill also creates a new federal crime to penalize attacks against U.S. service members on account of their service.

Hate crimes legislation enacted after King's assassination defined hate crimes as those carried out on the basis of race, color, religion or national origin. It also limits the scope of activities that would trigger federal involvement.

The proposed expansion would include crimes based on gender, sexual orientation, gender identity or disability. It eases restrictions on federally protected activities.

Some 45 states have hate crimes statutes, and the bill would not change the current situation where investigations and prosecutions are carried out by state and local officials.

But it would provide federal grants to help with the prosecuting of hate crimes and funds programs to combat hate crimes committed by juveniles.

The federal government can step in after the Justice Department certifies that a state is unwilling or unable to follow through on a purported hate crime.

The Associated Press contributed to this report.