WASHINGTON — New Hampshire Sen. Kelly Ayotte – a sought-after Republican vote in the debate over gun control – said Wednesday that she will not support a compromise to expand the background check requirements for private gun sales.

Republican Maine Sen. Susan Collins, meanwhile, took to the Senate floor to explain why she plans to support the proposed expansion.

Ayotte had voted last week to allow a series of gun control bills to move forward in the Senate. But hours before the Senate is slated to vote on a series of gun-related measures, Ayotte said she would not support the background check amendment negotiated by Democratic Sen. Joe Manchin of West Virginia and Republican Sen. Pat Toomey of Pennsylvania.

Ayotte’s decision is a major loss for backers of the amendment, who appear to be short of the 60 votes needed to advance the proposal in the Senate.

“I believe that restricting the rights of law-abiding gun owners will not prevent a deranged individual or criminal from obtaining and misusing firearms to commit violence,” Ayotte said in a statement. “While steps must be taken to improve the existing background check system, I will not support the Manchin-Toomey legislation, which I believe would place unnecessary burdens on law-abiding gun owners and allow for potential overreach by the federal government into private gun sales.”

Ayotte appears to be the only senator from New England poised to vote against the Manchin-Toomey proposal, which would require background checks for any private sales at gun shows or that were advertised online or in print.

Later Wednesday morning, Collins explained on the Senate floor why she planned to vote for the Manchin-Toomey amendment. Collins is one of a handful of Republicans who plan to support the proposed expansion.

Collins called the amendment “a reasonable, common-sense, thoughtful proposal.”

“Their bipartisan effort would strengthen the background check system without in any way infringing on our 2nd Amendment rights,” Collins said.

Collins said she was pleased with exemptions for gun transfers among family members, liability protections for private sellers, the proposed creation of a national commission on mass shootings and language explicitly prohibiting the creation of a national registry of gun owners.

The Manchin-Toomey amendment is the highest-profile amendment among a half-dozen expected to be voted on Wednesday evening. Others would ban assault weapons and high-capacity ammunition magazines, strengthen federal laws against gun trafficking and straw purchasers.

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