As former President Donald J. Trump tries to move past a speaker’s ugly insult of Puerto Rico at his Madison Square Garden rally, Democrats are working to elevate it — literally.

With Mr. Trump set to hold a rally Tuesday night in Allentown, Pa., which has a significant Puerto Rican population, the Democratic Party said it had invested in two digital highway billboards in the city that highlight the slight of the American territory.

The Democratic Party said the billboards shift back and forth between English and Spanish, displaying a Washington Post headline: “Trump rally speakers lob racist insults, call Puerto Rico ‘island of garbage.’”

The billboards appeared to serve two purposes — trolling the former president and appealing to voters who may have been offended by the rally on Sunday in Manhattan, where the comedian Tony Hinchcliffe dismissed Puerto Rico as a “floating island of garbage.”

About a quarter of Allentown’s population is Puerto Rican, according to census data, and Pennsylvania, a large and potentially decisive swing state, is home to about 470,000 Puerto Rican Americans.

The billboards started showing the message at midnight, according to the Democratic National Committee. Two more billboards displayed the message in Philadelphia, and nine carried it in Reading, Pa., the party said.

Puerto Rican residents in eastern Pennsylvania have responded to Mr. Hinchcliffe’s remark with shock and anger. And the Trump campaign has moved quickly to distance itself from the comedian, saying that his words did not reflect Mr. Trump’s views. (Mr. Hinchcliffe has suggested that Democrats lack a sense of humor.)

A spokeswoman for the Trump campaign, Vianca Rodriguez, responded to the billboards by saying Hispanic voters have been warming to the Republican Party under Mr. Trump because of his “plans to cut taxes, end inflation and stop the surge of illegal immigrants at the southern border.”

But Monica Guardiola, an executive director of the Democratic National Committee, said that Mr. Trump had a history of slighting Puerto Rico, noting that his administration imposed restrictions on emergency relief to the island.