Days after rousing audiences at a Florida speech he gave in which he evoked two powerful, inarticulate American icons: Martin Luther King Jr. and Mother Teresa, Joseph R. Biden Jr. gave the opening keynote address at the Fight for 15 Campaign Rally for Justice on Saturday in Washington, D.C.

A leading Democratic contender for the 2020 presidential nomination, Mr. Biden had a message for all those candidates who have yet to decide whether to run for president this year. He said those who are waiting to run should do so now, because without a president who can protect the middle class, the country will lose.

On taxes, the hawkish former senator said, the Democratic Party needs to establish that “the rich and the powerful are not enough” to underwrite all that the country needs.

On economic inequality, he noted, “The poverty rate has never been higher than 12 percent among the poorest 20 percent.”

“It is that inequality of opportunity that divides the rich from the poor in the United States of America,” said Mr. Biden.

In a gathering that featured Reverend Al Sharpton, NAACP president Cornell William Brooks,, and organizers of the Fight for 15 campaign, Mr. Biden tried to paint himself as a messenger who believes in economic justice.

Standing on stage with a group of young women whose jobs he would champion as president, he invoked the wishes of multiple election candidates including the first African-American female to be a major party’s presidential nominee, Al Gore.

“He tried to pass a comprehensive national child care plan during his time in Congress,” said Mr. Biden. “It didn’t happen. Maybe next time, he can get it done.”

He also likened his emphasis on employment to his travels through the Indian Subcontinent. “There’s an awful lot of young men and women in poverty on a boat. Poverty is an illness. It’s not an excuse, it’s not a reflection of laziness, it’s an illness. This is something we have to treat as our common enemy. It has to be destroyed.”