The summit, organized this year by US President Joe Biden's administration, was intended to convene leaders from across the Americas in Los Angeles to discuss common policy issues. As host country, the United States has the right to draw up its guest list.
In April, US Assistant Secretary of State Brian Nichols told reporters that authoritarian Cuba, Nicaragua, and Venezuela were unlikely to be invited. The high-level conference would instead focus on the Western Hemisphere's democracies, Nichols said.
While White House officials emphasize that the guest list is not yet finalized, even democratically elected leaders in the region are now warning that they won't attend the summit if not all countries are invited.
Notably, President Andres Manuel Lopez Obrador of neighboring Mexico has said that if other countries in the Americas are excluded, he would stay home in solidarity. "If they're excluded, if not all are invited, a representative from the Mexican government would go, but I wouldn't," Lopez Obrador said during his regular news conference last Tuesday.
Lopez Obrador's threats appear to have caused Washington to reconsider their position.
On Monday, Lopez Obrador said Mexico is "in dialogue" with the US.
"At least, they [United States] have acted in a respectful manner, there has not been a total, cutting rejection," he said.
The Summit of the Americas has taken place every three years since 1994 -- an opportunity for the US to shape policy and solidify partnerships in the region.
Cuba was excluded until 2015, when then-US President Barack Obama seemed to close a chapter of the cold war, sitting down with Cuban leader Raul Castro in Panama. The island returned to the conference in 2018 -- though the welcome was much chillier from the then-Trump administration, with then-US vice president Mike Pence criticizing its "tired Communist regime," and walking out during the Cuban Foreign Minister's rebuttal.
Cuba, so far largely frozen out by the current administration, is seeking an opportunity to protest directly to Biden increased US sanctions. Excluding the Communist-run island entirely this year -- as well as allied Nicaragua and Venezuela -- would be a pointed message on the part of the Biden administration.
The US has good reason to exclude some governments, says former US diplomat Eric Farnsworth, who worked on the inaugural summit of the Americas in Miami in 1994. "This particular forum is expressly reserved for democratically leaders and that's what the Biden administration is grappling with," he told CNN.
The US regards the elections of Venezuelan President Nicolas Maduro -- who is under US indictment for drug trafficking -- and Nicaraguan president Daniel Ortega as illegitimate.
Still, the absence of Mexico's Lopez Obrador would be a blow given the US and Mexico's shared border and interests. "The president of Mexico is a critical actor obviously in terms of US relations but also in terms of issues the US wants to discuss at the summit in Los Angeles, mainly migration," Farnsworth said.
Beyond Mexico, potential no-shows include the leftist leaders of Bolivia and Honduras. "If we are not all nations, it is not Summit of the Americas. The worthiest study of an American is America, " Xiomara Castro, President of Honduras wrote on her Twitter.
Guatemala's president has already said he won't go, after the US placed sanctions on the country's attorney general.
And geopolitical heavyweight Brazil is reportedly on the fence. According to Reuters, a spokesman for Brazil's Foreign Ministry has said no decision had been taken yet on whether President Jair Bolsonaro would attend. "The president's attendance is being studied and is not confirmed," the official told the news agency. Bolsonaro and Biden have never spoken to each other.
In tweets last week, Cuban Foreign Minister Bruno Rodriguez accused the US of excluding his country and Venezuelan Foreign Minister Carlos Faria hailed the intervention of Mexico's president.
Meanwhile, Nicaraguan leader Daniel Ortega has said he wouldn't go even if the red carpet were rolled out.
"We are not interested in being at that summit," he said, adding that the conference now is "dirty, muddy."