They got a boost from President Donald Trump himself, who tweeted Saturday that he would like to help maintain the event, which is bogged down in a dispute over costs.

The huge motorcycle rally -- set to take place on Sunday -- began in 1988 with fewer than 3,000 participants under the motto "We will never forget." The goal was to press for an accounting of those missing in Vietnam.

Over the years, it has grown into a rumbling combination of protest and parade, with tens of thousands of participants and many more lining the streets from the Pentagon to the National Mall to watch -- and hear -- the gleaming spectacle.

Riders end up at the Vietnam Veterans Memorial and the nearby Lincoln Memorial for speeches and a concert on the Sunday before Memorial Day.

Unlike Veterans Day in the United States, which honors all military veterans in November, Memorial Day -- on the last Monday in May -- is aimed specifically at remembering those who died while serving in the US armed forces, were prisoners of war or remain unaccounted for.

The event has the same name as a major US bombing operation against North Vietnam during the war.

Bikers kicked off the weekend of events with a "blessing of the bikes" at Washington National Cathedral on Friday.

Organizers however have announced that this year will mark the last national rally, citing difficult relations with the Pentagon -- where the riders line up to begin the rally -- over logistics and high costs.

"As always, the Pentagon is charging us with an outrageous bill for their services," the group's national president Joe Bean said in a letter to members.

"The organization will continue to bring awareness to the public, in years to come, with regional demonstrations," organizers said in a statement on their website.

Trump, who was on an official visit to Japan, offered his support.

"Can't believe that Rolling Thunder would be given a hard time with permits in Washington, D.C. They are great Patriots who I have gotten to know and see in action. They love our Country and love our Flag. If I can help, I will!" he said.

According to the Pentagon's POW/MIA Accounting Agency, more than 82,000 US troops remain unaccounted for in wars as far back as World War II.