Maduro, 50, was excluded from the pomp and circumstance of presidential inauguration ceremonies in Asuncion, the Paraguayan capital while fellow socialists and middle-of-the-road populists supped with multimillionaire President-elect Horacio Cartes as he was sworn in Thursday.
Analysts say the snub was payback for Maduro's alleged meddling in Paraguayan politics. He's accused of trying to incite Paraguayan military leaders to rise in defense of former socialist President Fernando Lugo after he was ousted in June last year.
Lugo was impeached by Paraguayan Congress and replaced with a caretaker government which held elections that brought the center-right Cartes to power. The impeachment, following 17 deaths in a land dispute, prompted the Mercosur regional pact to suspend Paraguay's membership and fast-track Venezuela's membership, which had been blocked by Paraguay.
Now, with Venezuela in and Paraguay out, Mercosur says Paraguay can come back. Asuncion views Cartes' inauguration as an opportunity to have its regional stature restored but isn't keen on returning to Mercosur.
Mercosur aides say Paraguay's return is a mere formality, but Asuncion says it won't join unless Venezuela is expelled. An early end to that stalemate is unlikely, but that isn't stopping Paraguay from snubbing Venezuela any it can.
The Paraguayan Senate voted against Venezuela's entry into Mercosur and declared Maduro persona non grata for his alleged interference in Paraguay's internal affairs. During a visit to Asuncion, as foreign minister to former President Hugo Chavez, Maduro was caught on video allegedly plotting with Paraguayan military leaders.
Until recently Argentina, Brazil and Uruguay were working in solidarity with Venezuela to try and keep Paraguay isolated, but the inauguration gave them an opening and everyone turned up for the party. Everyone except, that is, Maduro and his aides.
An early "fix" of the diplomatic conundrum isn't likely, analysts said.
In his inaugural speech, Cartes promised to wage war on poverty.
"I'm in politics to serve my people, improve the future of new generations and treasure our identity as a free, independent and sovereign people," he added.
Maduro made similar promises when he took over after his mentor, Chavez, died of cancer in March. Unlike Cartes in Paraguay, Maduro's task is made difficult by a constant struggle to balance economic pragmatism and Chavist populism, seen as crucial to approval ratings among Chavez loyalists.
Oil-rich Venezuela is battling recession for a third year, inflation is galloping and the security services are focused more on containing opposition to Maduro than on beating urban violence.
Venezuela recorded more than 21,692 homicides last year, a record, along with a surge in kidnappings, prison riots and random shootings.