If President Trump endorses gun-control legislation, it will take 13 Senate Republican votes to pass the measure, assuming the entire 47-member caucus of Democrats and Independents backs it.
Conservatives such as Sen. Ted Cruz (R-Texas) say any proposal that goes as far as the 2013 amendment sponsored by Sens. Joe Manchin (D-W.Va.) and Pat Toomey (R-Pa.) will face a backlash from the right, meaning 60 votes will be necessary to break a filibuster.
The Manchin-Toomey proposal, which is basis of current Senate negotiations, would expand back to include all sales over the Internet and at gun shows but exempt sales between family, friends and coworkers.
Here are the 13 Republicans crucial to passing gun-control legislation:
A brief synopsis of the issues and the players.