CÚCUTA, Colombia — The battle over the legitimate leadership of Venezuela — which has included rallies of thousands, international diplomacy and oil sanctions — is now focused on a single heavily guarded shipment of humanitarian aid.

Venezuela’s opposition, which has relished a month of victories in its effort to challenge President Nicolás Maduro and take over as the country’s legitimate government, brought the donated supplies of food and medical kits to the country’s border with Colombia.

Its goal was to bring the supplies into Venezuela, forcing a confrontation with Mr. Maduro, who has refused the help. This would cast Mr. Maduro in a bad light, opposition leaders said, and display their ability to set up a government-like relief system in a nation where the crumbling economy has left many starving, sick and without access to medicine.

But there was no dramatic confrontation.

Instead, Mr. Maduro’s administration erected a crude, but effective blockade across the border bridge with Colombia. The move brought the relief effort to a halt, and left the opposition and its leader, Juan Guaidó, at a standstill, aware that each passing day dampens its considerable momentum toward winning the trust of Venezuelans and the recognition of other governments. A delay could also mean reverting back to the status quo, in which Mr. Maduro retains control.