For the Venezuelan government, everything seemed to be falling in place.

Francisco Torrealba, a senior ruling party official, described being in an electoral command center in the country’s capital, Caracas, on Election Day last month, watching the computer monitors with confidence as the presidential vote neared its close.

The charts showed that a crucial party support base in Caracas had shown up in force.

The picture was much the same in other traditional government strongholds nationwide, he said. This assured officials that a combination of high turnout among loyalists and suppressing the vote for the opposition would propel Venezuela’s president, Nicolás Maduro, to a presidential election victory.

“We were calm,” Mr. Torrealba, a veteran lawmaker and a senior official of the ruling Socialist Party, said in an interview, describing the mood among government officials during the July 28 vote. “We did everything necessary to achieve a good victory.”

What happened next appears to have delivered a seismic jolt to the government’s expectations.

Vote tallies showed that the ruling party’s supporters in the public sector and poor neighborhoods had abandoned the country’s leader in droves, according to vote tallies obtained by the opposition. An electoral disaster loomed.

“We were betrayed, because they said they were going to vote for Maduro and what did they do? They voted for the lady,” said a ruling party activist in Maracaibo, Venezuela’s second largest city, speaking anonymously for fear of retribution.