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The gun-control advocacy group Mayors Against Illegal Guns, for example, has targeted Republican Sens. Jeff Flake of Arizona and Kelly Ayotte of New Hampshire, as well as Democrat Mark L. Pryor of Arkansas, for voting against the bill, while thanking GOP Sen. Patrick J. Toomey of Pennsylvania, one of the measure’s co-sponsors.

The legislation would have expanded background checks on firearms transactions to those conducted online and at gun shows. Right now, only federally licensed dealers are required to perform the checks.

Sen. Joe Manchin III, West Virginia Democrat and co-sponsor of the provision, has said he hopes to bring gun legislation back up for consideration later this year.