But in other matters, Mr. Rubio has taken a more muddled approach. He has castigated the Trump administration for its handling of Russia and threatened to vote down Secretary of State Rex W. Tillerson’s nomination, only to back away from the confrontation. After advocating a comprehensive immigration overhaul under the Obama administration, Mr. Rubio opposed a bipartisan immigration deal this month, saying that it did not do enough to bolster enforcement.

Mr. Rubio maintains that he has demonstrated independence from the G.O.P. by consistently defending Robert S. Mueller III, the special counsel investigating Russian meddling in the presidential election, and been openly critical of the federal government’s initial response to Hurricane Maria in Puerto Rico. His reaction to the Parkland shooting, in his view, has built on his longstanding gun policies, now informed by lessons law enforcement and schools officials have learned in the first few days since the massacre at Stoneman Douglas High.

On the Democratic side, a mood of cynicism tends to pervade discussions of Mr. Rubio: Representative Ted Deutch, a Democrat who joined Mr. Rubio in the Wednesday town hall and whose district includes Stoneman Douglas High, accused Mr. Rubio of favoring only “legislation that the N.R.A. can live with.”

Still, some Democrats welcomed Mr. Rubio’s apparent openness to new legislative options. Representative Charlie Crist of Florida, a Democrat and former Republican who was once Mr. Rubio’s bitter rival in a Senate primary, said Mr. Rubio’s comments offered reason for optimism.

“I’m encouraged by that, to be honest, and I hope that that evolution continues,” Mr. Crist said Thursday. “In the wake of an event such as this, that can happen.”

Mr. Crist, a onetime N.R.A. ally who now supports a ban on assault weapons, said much of Florida had undergone a similar reassessment of gun rights orthodoxy after two massacres in the state that targeted young people.

Pushing back on criticism from Mr. Deutch and others, Mr. Rubio fiercely disputed that he operates with great deference to the N.R.A. He countered that he is focused on practical ideas that might stand a chance of becoming law, and said the assault weapons ban favored by Parkland families is not among them.