Rolling Thunder, after 31 years, announces its last Memorial Day ride to take place in 2019

The group cited rising costs and "increased harassment" from police.

It's been a Memorial Day weekend tradition for 31 years and is a demonstration to bring attention to and demand accountability for prisoners of war and those who are missing in action.

In 2018, the ride drew more than half a million participants, but rising costs and "increased harassment" from the Pentagon and police in Washington led the group to decide the ride was no longer feasible, according to its founder and executive director Artie Muller.

Muller said in a statement that the ride's supporters were also "diverted and prevented" from entering one of the Pentagon parking lots where the event stages.

In a statement to ABC News, the Pentagon said it was prepared to support the 2019 ride, but referred inquiries about the status of the 2020 ride to Rolling Thunder.

"The department supports the peaceful, lawful exercise of American citizens' First Amendment rights, and remains focused on ensuring the safety and security of the demonstrators and the Pentagon Reservation," Pentagon spokesperson Sue Gough said. "The department is prepared to support the 2019 Rolling Thunder ride, as we have for the last 31 years."

According to Muller, staging costs for the 2018 ride reached $200,000, and there wasn't enough funding through event product sales and corporate sponsorship.

"Financial factors are draining the organization funds if we continued this major costly annual event in Washington," Mueller said.

President Donald Trump attended the 2016 ride, even speaking to attendees from a stage at the edge of the Reflecting Pool on the National Mall.

Joe Chenelly, the executive director of AMVETS, urged participants on Twitter to not cancel their hotel reservations for 2020.

"We have been concerned this was coming for some time, so we've prepared," he said. "We will not let this vital demonstration in our nation's capital end."

Muller hopes that supporters will become involved in the 90 Rolling Thunder state chapters across the country, which are starting their own 2020 Memorial Day Weekend demonstrations.

The final ride in the nation's capital is scheduled for May 26, 2019.