"It's just another example of how Manchin was willing to say one thing in order to fool West Virginia voters, but is just another Obama supporter when he crosses (sic) to Potomac."
In an article in the conservative magazine National Review, Lucas also questioned whether Manchin was "sucking up" to national Democratic leadership.
"And when you're sucking up to Harry Reid and Barack Obama, you are spitting in the face of West Virginians," Lucas is quoted as saying in the article.
That's not the reaction Manchin said he received from Republicans in West Virginia about the measure. He said he doesn't think rank-and-file Republicans agree with party leadership and encouraged everyone to the read the amendment before forming an opinion.
National pro-gun organizations blasted Manchin and Toomey.
The Gun Owners of America called the amendment the "See a shrink, lose your guns" bill, regarding provisions that address mental health. The National Rifle Association has given Manchin an "A" rating, but it also voiced displeasure Wednesday.
"Expanding background checks at gun shows will not prevent the next shooting, will not solve violent crime and will not keep our kids safe in schools," the NRA posted on its website.
Keith Morgan, president of the West Virginia Citizens Defense league, declined comment.
Manchin asked NRA members to read the measure and encouraged the organization to put the full amendment on its website once the information is available. He said the full amendment would be available online today.
Nationally, President Barack Obama, New York City Mayor Michael Bloomberg and others congratulated Manchin and Toomey on the background check expansion. Obama said he didn't think the changes went far enough but were a genuine sign of bipartisan cooperation to reduce gun violence.
Other members of the West Virginia congressional delegation gave mixed reviews.
Manchin said he spoke with all of them to let them know he was announcing the amendment, and hoped they would reserve judgment until reading it.
U.S. Rep. Nick Rahall, D-W.Va., said in a statement it was premature to discuss how the bill would fare in the House.
Rep. Shelley Moore Capito, R-W.Va., said everyone shares a desire to protect children but didn't say whether she supported the amendment.
"However, we have existing laws on the books that aren't being enforced and punishing law-abiding citizens or creating ineffective laws is not the answer," she said in an emailed statement.
Sen. Jay Rockefeller, D-W.Va, commended Manchin and Toomey.
"I'm hopeful that it will help clear the way for the Senate to fully consider gun safety legislation and hold a vote," he said in a statement from a spokesman.
Senate Majority Leader Harry Reid, D-Nev., has called for action on a larger bill concerning gun rights today. Manchin said Wednesday he expected his amendment to be the first proposed.
He thanked Toomey for his help. Toomey, who Manchin called one of the most conservative Republicans in the Senate, said the background check expansion is a "common-sense issue" and not about gun control.
Although Toomey said he knew some Senate and House Republicans were in favor of the changes, he told CNN, "I don't know in the end how many will support it.