As the dust settled on Monday and the crowds went home, opposition lawmakers were urging calm. They said they would not fight back even if the new constituent assembly were to force them from their chamber, as many radical leftists are now urging the assembly to do.

“If they come with arms to take control of the national legislative palace, we can find another place to hold our sessions,” Henry Ramos Allup, a prominent opposition lawmaker who until recently served as head of the National Assembly, told reporters.

Pressure was mounting on Mr. Maduro as well. On Monday afternoon, the United States Treasury Department added him to a growing list of Venezuelan officials facing sanctions, freezing any American assets the president owns and forbidding Americans to do business with him.

But while the White House had encouraged the Venezuelan opposition in recent days, the sanctions were far less severe than the crippling economic penalties it had threatened against Mr. Maduro before the vote on Sunday. Mr. Maduro is now one of only four heads of state to be sanctioned this way, including Bashar al-Assad of Syria, Kim Jong-un of North Korea and Robert Mugabe of Zimbabwe.

“Yesterday’s illegitimate elections confirm that Maduro is a dictator who disregards the will of the Venezuelan people,” Treasury Secretary Steven Mnuchin said in a statement.

The limited American response left many of Mr. Maduro’s rivals pondering their future under the constituent assembly.