CÚCUTA, Colombia — As the humanitarian aid at the heart of a Venezuelan border standoff remained shut in warehouses on Sunday, and with President Nicolás Maduro’s blockade still intact, it became clear that the opposition leaders trying to oust him had little in the way of a Plan B.
Juan Guaidó, the top opposition official, and his allies had hoped that forcing the badly needed food and medicine inside Venezuela would represent a moment of irreversible collapse in Mr. Maduro’s authority. Instead, just one aid truck made it through on Saturday, the deadline set by the opposition to end the impasse, and Mr. Maduro easily fended off the biggest challenge to his power since Mr. Guaidó swore himself in as the country’s rightful leader last month.
Clashes between opposition protesters and forces loyal to Mr. Maduro, which have left four dead since Friday, continued into Sunday, threatening the image of Mr. Guaidó’s nonviolent movement. The military officials crucial to keeping Mr. Maduro in power largely resisted Mr. Guaidó’s call for mass defections, with only about 150 deserting. And even Mr. Guaidó’s own fate remained unclear: After he slipped over the border into Colombia on Friday, disobeying a travel ban, it was anyone’s guess if Mr. Maduro would allow him to return.
“It was one of the outcomes we had imagined, but it wasn’t the one we wanted,” said Armando Armas, an opposition lawmaker, adding that it was unlikely the organizers would try to get the aid through again soon. “We can’t expose our people any more. The entry of humanitarian aid can’t be the trigger of a wider conflict.”
But while Mr. Maduro prevailed in this border showdown, conditions inside the country remain deeply unfavorable to him. He is still immensely unpopular within Venezuela, where he has overseen one of the most catastrophic economic disasters in Latin American history, a calamity that has led a tenth of the population to leave the country, largely because of shortages of food and medicine. Further damage was done to his image on Saturday as he denied aid to suffering Venezuelans.
Mr. Guaidó, for his part, has galvanized his country, and more than 50 other nations have recognized his claim to the presidency. The Trump administration, a vocal supporter of Mr. Guaidó, has issued crippling sanctions against Mr. Maduro’s state oil company.
Vice President Mike Pence will arrive on Monday in Colombia, where he plans to meet with Mr. Guaidó and intends to announce “concrete steps” to further pressure Mr. Maduro’s government. That could include more punishing sanctions.
Still, even if few doubted that the opposition’s resolve would be lost over its failure to deliver the food and medicine, Saturday seemed like a turning point, with new talk of a need for foreign intervention.
Mr. Guaidó, a 35-year-old who had emerged onto the national stage only in recent months, had sought a political weapon in the arrival of international aid donations. He announced Saturday as the day of an “aid avalanche,” in which his supporters would defy the president and break his control of the borders. The presidents of Colombia, Chile and Paraguay joined the effort, along with the British billionaire Richard Branson, who flew to the border to attend a concert where a lineup of Latin pop musicians urged that the aid be allowed through.
Yet by day’s end, the avalanche was more like a drop in the bucket.
Some of the supplies, instead of reaching Venezuelans in need, had been burned after crossing from Colombia. Mr. Maduro, seeing the aid as undermining his authority, had declared that his country was not a beggar and did not need it. Government forces and allied groups backed up that message with violence, bringing condemnation from the United Nations on Sunday.
Thwarted at the border, the opposition began rallying around the banner of foreign action to topple Mr. Maduro. Ahead of the meeting with Mr. Pence and other regional leaders in Bogotá on Monday, Mr. Guaidó wrote on Twitter that “we must keep all options open for the liberation of our homeland.” Julio Borges, another opposition leader, said he would ask for the use of force at that meeting.
President Trump has raised some hopes for such an intervention, saying that “the twilight hour of socialism” has arrived in the Western Hemisphere.
But “the U.S. rhetoric was over the top,” said Charles S. Shapiro, a former ambassador to Venezuela, adding that the statements may have led some in the opposition to believe that military assistance would be on the way if Mr. Maduro blocked the aid.
As those shipments were stymied, images of violence marked Colombia’s border bridges on Sunday. Near the Santander Bridge, where aid had been burned the day before, dozens of young men hurled stones at Venezuelan security forces and erected barriers.
Others stalked the streets of a nearby neighborhood in search of ingredients to make homemade bombs and masks to ward off tear gas.
“Yesterday, we brought humanitarian aid, we brought flowers and flags, and we got bullets instead,” said Delbert Rondón, a 34-year-old who had left the western city of Mérida seeking medicine. “I’m here under the bridge helping the boys — I pass them rocks, rags, bottles of water, vinegar. We have to help them because they are the resistance.”
Those scenes, however, were ones that the opposition had sought to avoid.
Major protest movements in 2014 and 2017 started off as peaceful demonstrations, but as the weeks wore on, they were overtaken by groups largely composed of young men who clashed with security forces in the streets. In 2017 alone, the clashes resulted in the deaths of more than 100 people on both sides.
Mr. Guaidó’s movement had appeared to avoid street violence and requests for foreign military intervention.
After declaring himself president on Jan. 23 to the cheers of crowds, he had amassed hundreds of thousands of supporters on the streets of Caracas, the capital, and other cities in largely peaceful rallies. His strategy was rooted in a mix of civil disobedience and international pressure, urging Mr. Maduro’s military to abandon him, rather than seeking an outside military campaign to topple him.
“The nonviolent character of these protests is absolutely key in making a difference,” said David Smilde, a senior fellow at the Washington Office on Latin America. “When there’s fighting, it doesn’t go well for a simple reason: Once these protests get violent, they will be no longer massive.”
Mr. Smilde added that the more the opposition requests outside military intervention, the less likely it is that Venezuelans will demonstrate in the streets, believing foreign help is near.
A march on Sunday on the Colombian border was meant to be a model for others to come. Organizers said the plan had been for protesters to walk alongside trucks filled with humanitarian aid across three bridges, persuading soldiers on the other side to let the aid across with appeals that the families of the armed forces suffer the same shortages as most Venezuelans.
The marchers received blessings by a priest at an encampment at sunrise, and by early afternoon thousands had gathered with white roses and had begun linking arms near the Tienditas Bridge. Organizers planned to use a tow truck to remove shipping containers left as an obstacle by Mr. Maduro’s government.
At a pedestrian footbridge farther south, cheers erupted as aid trucks began to approach the border, with hundreds of young Venezuelans sitting atop the shipments.
But then Venezuelan national guard soldiers fired tear gas canisters toward the cars. Many of the protesters stepped off the trucks and rushed the soldiers, throwing stones. Soon, large crowds were tossing rocks up to the stone throwers on the bridge, as opposition activists and Colombian national police officers watched on.