President Donald Trump’s upcoming five-nation Asia tour is expected to strengthen ties with regional allies, but those gains could be diminished if the U.S. leader misses a key summit on the last leg of his trip.

A major forum for political and economic cooperation among its 18 member countries, the annual East Asia Summit takes place on Nov. 13 to 14 in the Philippines this year. Trump will be in Manila on Nov. 12 but is expected to leave before the event, which isn’t included in a schedule released by the U.S. State Department.

Under former president Barack Obama, the East Asia Summit became a pillar in Washington’s Asia relations. In 2011, the U.S. acquired membership under Obama’s strategy of pivoting American foreign policy focus toward the Asia-Pacific region. Since then, the president attended every year except for 2013 when a U.S. government shutdown occurred.

Trump’s Asia tour is the longest trip to the continent by a sitting U.S. president this millennium but missing the East Asia Summit is still a big deal for local leaders, experts say.

“The U.S. decision to join EAS was a milestone in U.S. engagement in Asia and a tradition that should be followed,” said Thomas Hubbard, a former U.S. ambassador to South Korea. “I hope Asians will focus on the unusually long time he will spend in Asia rather than on the one meeting he will miss.”

Donald Tusk from the European Council, Canadian Prime Minister Justin Trudeau, and United Nations Secretary-General Antonio Guterres are all expected to attend this year’s meeting.

“Many Asian nations — rightly or wrongly — look at the U.S. president’s attendance at the EAS as a barometer of America’s enduring commitment to the Asia-Pacific, and arguably will even more so in the aftermath of Obama’s rebalance,” explained Ashley Townshend, acting director of the foreign policy, defense and strategy program at the Sydney-based United States Studies Center.

An op-ed in the Washington Post citing unnamed officials said there were concerns the trip’s length could make the president “cranky, leading to unpredictable or undiplomatic behavior.”

Trump’s absence could ultimately hurt U.S. influence in Asia, experts said.

“Trump’s decision not to participate in the East Asia Summit will reinforce regional perceptions that he is skeptical of the value of multilateralism and of a United States that is scaling back its engagement in Asia,” said Meredith Miller, a former deputy director at the U.S. State Department’s East Asia and Pacific bureau.

A lack of participation also signals that “he is out of touch with what matters to America’s regional partners,” said Townshend.

More concretely, Trump will miss a chance to add input on security challenges critical to U.S. interests. East Asian Summits usually produce a statement that touches on hot-button topics such as the South China Sea and North Korea.

“Sitting down with the leaders there is an opportunity for the president to make his points on these issues,” Walter Lohman, director of the Asian Studies Center at conservative U.S. think tank The Heritage Foundation, said in a recent note. “It would help shape a consensus that can then be used in other diplomatic settings.”

The U.S., of course, will still send representatives to the event. But the commander-in-chief’s absence could affect negotiations.

“The U.S. will be disadvantaged when it comes to negotiating with other leaders on the EAS statement, which is likely to address issues central to Trump’s agenda in Asia,” said Miller, who is currently vice president of the East Asia & Pacific practice at strategy firm Albright Stonebridge Group.

Trump’s anticipated no-show could also leave Beijing with greater room to fill the leadership void as the world’s second-largest economy looks to extend its sphere of influence in Asia and beyond.

A decision to forego participation in the EAS will be “disappointing,” said David Adelman, former U.S. ambassador to Singapore, noting that the summit is regionally important because of its size and composition. Trump’s absence could “add to the growing narrative that [his] ‘America First’ agenda may work at cross-purposes with traditional U.S. leadership in the region.”

Media reports say Trump is expected to send Secretary of State Rex Tillerson in his place. But the former Exxon Mobil CEO’s presence may be of little solace to Asian leaders seeking clarity on U.S. policy.

The commander-in-chief has frequently undercut Tillerson’s diplomatic efforts so “now when Tillerson tries to conduct negotiations, people know he won’t have the White House behind him,” said Anne-Marie Slaughter, a former director of policy planning at the U.S. State Department.

“In the end, the White House will beat the State Department every time,” she told CNBC Thursday, adding that she didn’t expect Tillerson to remain in his post for the duration of Trump’s term.