When asked why he chose to ride the subway on Wednesday, the mayor offered a simple reply — “It’s the easiest way to get where I’m going” — before glancing back at his briefing. He was headed to the Museum of Natural History, a spokesman said.

His C train ride came on the same day that Sal Albanese, a former city councilman who has begun a long-shot challenge to the mayor in the Democratic primary, held a news conference outside Mr. De Blasio’s gym, with a spare MetroCard in hand, calling on him to use mass transit instead.

The mayor, in defending his preference for his S.U.V. transport, said in an interview with WNYC earlier this month that “the issue is not cheap symbolism,” suggesting he had no desire to stage a subway ride for show. He added that his vehicle was “fuel efficient.”

As delays have racked up and many riders have directed their ire toward the mayor, Mr. de Blasio has been quick to deflect the blame toward Gov. Andrew M. Cuomo, who controls the transit authority and has faced growing pressure to fix the system.

“The M.T.A. is run by the State of New York, not the City of New York,” Mr. de Blasio said in an interview with WNYC in May. “But we’re going to do all we can to push the state to invest in the subway system.”