The seven-foot-tall metal fencing that has sealed the perimeter of the U.S. Capitol grounds and fortified the Supreme Court across the street is temporary. But it portends lasting change likely to come: In the capital city, there will be more hardening, more barriers, less openness, less access.

For 25 years, Washington has grown ever more conspicuously guarded, first with the bollards and concrete jersey barriers that appeared after the Oklahoma City bombing in 1995, then the elaborate security protocols that swept federal properties after Sept. 11, 2001. Then there were heightened fears of what could harm the nation’s first Black president, followed by new worries that everyday public spaces — plazas, parks, farmer’s markets — could be targets as much as the monuments were.

Now, in the heart of the federal city that Pierre Charles L’Enfant planned in 1791 to embody democracy itself, the broad avenues that give view of the Capitol from all directions lead to a barricaded compound. Its lawns are patrolled by National Guard troops. The Capitol hasn’t truly been open to the people for some time, certainly not in the way L’Enfant envisioned, and it will be even less so now.

“It breaks my heart that I can no longer access a building that has meant so much to me during my lifetime,” said Kenneth Bowling, a historian at George Washington University. “It’s the reason I became a historian, the reason I became a Constitutional historian of the American Revolution.”

Mr. Bowling, now 80, used to take a 25-cent train ride down from Baltimore as a teenager to walk the halls of the Capitol. A tour guide who sensed his wonder showed him the vault two floors below the Rotunda where they stored the Lincoln catafalque, the wooden platform used since 1865 to support the coffins of Americans lying in state at the Capitol.

That was decades before the construction of the $600 million underground Capitol Visitor Center, which since 2008 has shunted tourists through security checkpoints, into curated exhibits and on tightly controlled tours. That was before it became impossible to simply walk up the western steps of the Capitol to sit and watch the sun set across the National Mall.

“As to what the founding generation would have thought,” Mr. Bowling said, “obviously Washington and L’Enfant expected the citizenry to come to this great Capitol.”