Sen. Martin Heinrich, D-N.M., said he opposed the concealed-carry bill because states have different standards by which permits are issued, and it's not "realistic" to make all of them equal.
Asked why he didn't oppose the legislation on those grounds four years ago, Heinrich said he'd have to go back and refresh his memory, but conceded that "I think the approach that's being taken now, especially given the challenges that we have with mass shootings and gun violence in this country, is not the thoughtful way to go about this."
Sen. Tom Udall, another New Mexico Democrat, confirmed through a spokeswoman he also doesn't support the bill this time.
Democratic Sens. Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota and Jon Tester of Montana said they were studying the measure, a top legislative priority of the National Rifle Association and championed by Senate Majority Whip John Cornyn, R-Texas.
"I really haven't had a chance to take a look at it," said Tester. "I'm going to tell you I'd be open to it, but I gotta see it. I gotta look at it."
Sen. Joe Donnelly, D-Ind., told McClatchy to contact his press office, which did not return requests for comment.
In the House, Republican leaders anticipated overwhelming Democratic opposition by packaging the concealed-carry bill with two other measures.
One would put more mechanisms in place to ensure federal background checks are administered without error, while another would require the Justice Department to rule on the legality of "bump stocks," an accessory that can make a semiautomatic rifle more lethal.
Though a few Democratic centrists are expected to back the bill Wednesday, most Democrats have not been swayed.
Rep. Richard Hudson, R-N.C., the sponsor of the House bill, said the three bills were collectively "a great win for conservatives" and predicted they would pass the Senate as a package, too.
Cornyn, however, told reporters Tuesday that he expected to move the measures separately to at least allow the background check measure, which he also sponsored, to advance.
"I support both of the bills, but I recognize that when you put them together, it makes it harder for us to do what we can do and can do now, and need to do now, which is pass (the background check bill)," Cornyn said. "I'm willing to separate those two out."
"We'll see," Hudson said when told support was eroding from the seven Democrats who voted "yes" on concealed-carry reciprocity in 2013. "We'll see."
(Brian Murphy and Andrea Drusch of McClatchy's Washington bureau contributed to this report.)
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