Biden is making a bid to unite the hemisphere. Attendance is an issue.

President Joe Biden’s high-stakes Summit of the Americas was meant to reaffirm U.S. ties to its own neighborhood while tackling thorny issues of economic growth and migration. But one of the most important neighbors won’t be attending, raising substantial questions about the relevance of the gathering.

Mexican President Andrés Manuel López Obrador, a key player in Biden’s plan, announced Monday that he will skip the summit, a boycott that could prompt other nations to follow suit. The move left White House officials scrambling to set the agenda, not to mention the guest list. And it cast doubt on the chances of success at a moment when a rise in migrants at the U.S. border has become a searing political flashpoint.

After Donald Trump antagonized much of the region during his presidency, Biden has expressed hopes of stabilizing the gathering of nations — which stretches from Canada in the north to Chile in the south – while also countering China’s growing influence in the region. But the challenges are growing.

López Obrador had served as ringleader of mostly leftist leaders pushing the U.S. to invite the autocratic nations of Cuba, Nicaragua and Venezuela to the gathering, something the Biden administration has resisted because of political optics and their human rights records. And other leaders, including from Guatemala, Honduras and El Salvador — the three main drivers of migration to the U.S. — previously indicated they’d also abstain from attending if those nations are not invited.

“The region is in serious economic distress, and its economic struggles are eroding support for democracy,” said Benjamin Gedan, acting director of the Wilson Center’s Latin America Program. “Biden’s election generated high expectations for U.S. reengagement in the region, and so far, most everyone has been disappointed.”

The summit, which begins today, is the ninth such gathering since the 1994 summit was held in Miami, the last time the U.S. hosted the event. But the backdrop of this gathering may prove more chaotic than those of the past.

López Obrador had hinted at a boycott for weeks, though many in the Biden administration didn’t expect him to follow through. Publicly, White House aides downplay the possible absences of Mexico and other nations following suit. But privately, there has been growing frustration within the West Wing that the back-and-forth soap opera over the guest list has been the defining – and only – breakthrough conversation about a summit overshadowed by recent headlines, including a spate of mass shootings, inflation and the war in Ukraine.

“There's always questions about the invites, there's always questions about who is coming and who is not, but we should also talk about and focus on what the purpose of this meeting is,” White House press secretary Karine Jean-Pierre said this week. “I think if you've been following this administration for the past year and a half, one week is not the eleventh hour when it comes to how things move. And so that is a lifetime away for us as a White House.”

Mexico’s foreign minister will attend the summit instead, and López Obrador said Monday he intends to visit the White House next month.

The White House in the meantime has moved forward with its plans for the gathering. In total, 23 countries will be represented at the summit.

Biden will outline a cooperative economic vision for the region, as well as propose plans to tighten supply chains, particularly for medical supplies to prepare for future pandemics. He is expected to announce a food security plan and task Vice President Kamala Harris to lead a climate change partnership with Caribbean nations.

The main event will likely be the discussions on migration. Biden will stress that every nation – not just the U.S. and Mexico – has experienced irregular migration during the pandemic and will reveal a new plan emphasizing responsibility-sharing and economic support throughout the Western Hemisphere.

Migration to the southern border has increased to its highest levels in decades within the past year, and officials warn that those trends are expected to continue even though pandemic-era restrictions have curtailed access to asylum.

Other priorities, per officials, will be creating a pathway for the region to transition to clean energy as well as move toward a more digital future. And, underscoring a Biden campaign theme, the White House will use the moment to advocate for the vitality of democracies in an effort to blunt the rise of autocratic governments.

But the push runs into some political realities, coming as Biden considers a meeting in Los Angeles with Brazilian President Jair Bolsonaro, a nationalist who has not committed to honoring the results of his nation’s election later this year if he were to lose. And despite nixing the presence of Cuba, Nicaragua and Venezuela, the White House is also in the planning stages of a possible meeting with Saudi Arabia’s autocratic leader later this summer.

Domestic factors will, of course, impact any of the foreign policies Biden tries to implement while in California. Beyond the fervor of migration, economic headwinds may make it difficult for the president to unveil significant trade deals, while the complicated politics inherent to Florida make it hard for Biden to adjust U.S. diplomacy toward a country like Cuba.

“Historically, the U.S. gets in trouble in Latin America when it is absent, negligent, not engaged. Problems fester and grow into crises,” said Bernard Aronson, a former assistant secretary of State for Inter-American Affairs and former U.S. special envoy to the Colombian peace process.

“It's not too late, by any means,” Aronson continued, “but if you look at what Biden’s got on the plate elsewhere -- an administration can only do two or three big things at a time. He’s got Ukraine, Iran, China, inflation. I don’t think he has a lot of bandwidth available given what’s on his plate.”

Electoral politics may also come into play. Once the summit gets underway, some of Biden’s fellow Democrats could turn on him — they already have over his recent limited relaxation of Cuba and Venezuela policies — in an effort to shore up their moderate backers.

And Biden administration officials are bracing themselves for leaders to use the platform to decry U.S. regional policies, perhaps even to the president’s face – while other leaders, of course, will make their protests known in their absence.

Nahal Toosi contributed to this report.