This fascinating article in the Washington Post explains how gun politics, and pressure through the NRA’s rating system, is affecting the D.C. vote:

The immediate issue in the House is the parliamentary rule for the bill. Such a rule — which sets out the terms for debate and possible amendments — usually must be agreed upon before taking up the bill itself.
Supporters of the vote bill had assumed Democrats would use their majority power to pass a rule that would bar gun amendments.
But in an unusual move, the NRA told lawmakers that it might score their votes on the rule for the D.C. bill, Norton said. That means representatives could be recorded as casting an anti-gun vote if they approved a rule blocking amendments on the D.C. vote bill.