Thousands of U.S. military veterans and their supporters rode through Washington Sunday in their annual motorcycle rally for American troops listed as missing in action abroad.

For 17 years, this special parade of bikers from across the country has made its voice heard in the nation's capital. On this Memorial Day weekend, when America remembers its war dead, these veterans want to keep alive the effort to learn the fate of all those still unaccounted for.

"It is very important for us to bring them home, whenever possible," said Artie Muller, the head of Rolling Thunder Incorporated, the veteran's group that runs these annual rallies. "This is what Memorial Day is about: remembering those who gave their lives for their country, those who were left behind as POW-MIAs [a prisoner of war still unaccounted for], and those who were left behind as MIAs [a soldier missing in battle]."

Vietnam War veterans form the core of Rolling Thunder, and recently the organization voted to back President Bush in his bid for re-election.

It was a significant endorsement for the president, given the fact that his likely opponent Senator John Kerry served in Vietnam. After meeting privately at the White House with the Rolling Thunder leadership, Mr. Bush addressed the rally by telephone.

"I thought you were going to offer me riding lessons," he said. "Turned out you offered me Rolling Thunder's national endorsement for my re-election and I want to thank you for your support."

The president also thanked the group for its determination to get answers for the families of those listed decades ago as missing in action.

"I know that your work and your persistence and your diligence means a lot to relatives all over our country who wonder about their loved one, the fate of their loved one," said Mr. Bush.

Mr. Bush began the holiday weekend by taking part in the dedication Saturday of a new memorial honoring those who served in World War II. On Monday, he will speak at Memorial Day ceremonies at the military's Arlington National Cemetery.