The major gun control organizations, propelled by funding from supporters like Michael R. Bloomberg, the former New York City mayor, and grass-roots networks across the country, have helped enact new laws — mostly in Democratic-controlled states — and, for the first time in 25 years, passed a significant gun control bill in the House.

But the gun lobby’s structural advantages, built over decades and defended by President Trump and congressional Republicans, remain in place: an N.R.A. budget that dwarfs what even Mr. Bloomberg has spent, a Republican Senate majority disinclined to consider gun-control legislation, and a base of primary voters for whom the N.R.A.’s endorsement is a critical seal of approval.

The net effect is a playing field on gun issues that is far more level than it has been since N.R.A.-backed Republicans took over Congress in 1994, sparking one of the country’s most bitter, partisan culture wars.