With the political sands shifting in South America, the Bolivarian revolution of the late Hugo Chávez faces a major dilemma heading into crucial mid-term elections on Sunday.

Opinion polls show President Nicolás Maduro is set to lead his United Socialist Party of Venezuela, or PSUV, to its worst electoral defeat since before Chávez first won power in 1998.

Many voters, angry with a collapsing economy and stratospheric levels of violence, look set to abandon it.

Faced with the prospect of losing control of the 167-seat national assembly, Maduro and his lieutenants have been making worrying noises that they might not accept the result should it go against them, promising to “go to the street to fight with the people” if the PSUV is defeated.

Such rhetoric is part of what the main opposition alliance has denounced as “the dirtiest election campaign of the last 17 years”.

Before its final rally in Caracas on Thursday night the United Democratic Table, or MUD, presented what it said was evidence of official efforts to steal the election.

The most serious accusation was levelled against the Chavista-dominated national electoral council for placing a small Chavista party, MIN-Unidade, alongside the similarly named MUD on the ballot paper. It also allowed the MIN to use the same colours as the MUD in what opposition figures claim is a clear attempt to confuse less literate voters.

Several years ago Chavismo would have brushed off such complaints safe in the knowledge that regional allies would insulate it from criticism from the US and Europe.

But after more than a decade dominated by populists and left-wing parties, the election victory of right-winger Mauricio Macri in Argentina last month means South America's political make-up is shifting.

Suspension call

Macri has already called for Venezuela to be suspended from the

Mercosur

trade bloc for violating its democratic principles.

The incoming Argentine president believes actions such as the continued detention of Venezuelan opposition leader Leopoldo López and the shooting dead of another opposition politician at an election rally justify immediate suspension.

So far the other three members of Mercosur – Uruguay, Paraguay and Brazil – say they are not persuaded by the case for suspension.

On Friday Macri made his first visit to Brazil since his victory and his call for Venezuela's suspension was discussed at his meeting with Brazil's president Dilma Rousseff in Brasília.

Earlier this week she said the bloc’s democratic clause “had to be applied to determined facts, not generically”.

That was a rebuff to Macri, but also a warning to Caracas that, should tomorrow’s vote be seen to be unfair and the result disputed, the incoming Argentine president’s case for suspension will be strengthened.

Humiliating

Such a sanction would be deeply humiliating for Maduro and Chavismo, which has always boasted of its popular support and placed great store on Latin solidarity, being at the forefront of setting up several new regional organisations.

It celebrated its entry into Mercosur in 2012 with great fanfare and to be suspended so soon afterwards would only deepen the sense that the Bolivarian revolution has lost its way since the death of Chávez in 2013.

Despite Rousseff’s current stance, there are clear signs that Brazilian concerns over the deteriorating situation in Venezuela are mounting.

There was anger in Brasília after a former supreme court justice and minister was vetoed by Caracas from heading the regional Unasur organisation's electoral monitoring mission. That prompted Brazil's electoral court to scrap its own plans to monitor tomorrow's vote.

Unusually, Brazil’s foreign ministry also released a note this week saying it would “accompany with attention” the election and demanded that it “take place within the norms of democracy”. It followed a personal letter sent by Rousseff to Maduro said to have listed her concerns over the political situation in the country.

Should these Brazilian signals help ensure Sunday’s vote runs smoothly and produces a result acceptable to both sides, moves in the region to isolate Venezuela will likely lose steam. But if not, Macri will arrive at his first Mercosur summit in Paraguay on the 21st with his Venezuelan position reinforced.

His host, Paraguay's president Horacio Cartes, might emerge as his first ally. On Wednesday he met Lilian Tintori, wife of the jailed Venezuelan opposition leader Leopoldo López in a clear diplomatic signal to Caracas of rising regional concerns.

The days when the Bolivarian revolution could count on having its actions indulged by its neighbours look to be drawing to a close, further limiting its room for manoeuvre at home.