Both Sen. Bernard Sanders and Libertarian Party presidential nominee Gary Johnson teed off Thursday on Donald Trump at a gathering of Hispanic political leaders in Washington, D.C., as organizers lamented the absences of both Mr. Trump and likely Democratic presidential nominee Hillary Clinton from the conference.

“I firmly believe that Donald Trump will not become president of the United States for a number of reasons, but at the top of the reasons is the fact that the American people in 2016 are not going to vote for a candidate who insults Mexicans and Latinos and Muslims and women and African-Americans,” Mr. Sanders said at the National Association of Latino Elected and Appointed Officials (NALEO) conference.

Mr. Sanders, Vermont independent, has not formally conceded to Mrs. Clinton in the race for the Democratic nomination, but has indicated he will do whatever he can to stop Mr. Trump from winning the White House.

Mr. Johnson, meanwhile, told the crowd that immigration is a “wonderful thing,” saying it should be easy for someone who wants come into the country and work to get work a visa.

“I absolutely reject the notion that we should deport 11 million undocumented workers,” said Mr. Johnson, a former New Mexico governor. “That is just incendiary. That is a misunderstanding of this issue in a really big way. And building a fence across the border is misunderstanding this issue in a really big way.”

Mr. Trump repeatedly has touted a proposal to build a wall along the U.S.-Mexico border that he says Mexico will pay for.

Breaking precedent from the past two presidential election cycles, both Mr. Trump and Mrs. Clinton declined invitations to speak at the conference. Both major-party nominees addressed the conference in 2008 and 2012, and Mrs. Clinton herself spoke there in 2008.

“The absence of the Democratic and Republican presumptive nominees has created great consternation among the bipartisan NALEO membership,” the group said in announcing part of the schedule for the conference, which runs until Saturday.

Mr. Trump was preparing for an overseas trip to Scotland for an event at one of his golf courses. The Clinton campaign said the candidate is indeed focused on winning over Latino voters in the election, and said the campaign will continue to ramp up efforts to engage the Latino community as the general election contest moves forward.

Mrs. Clinton’s campaign on Thursday also pointed out that she will be addressing members of the League of United Latin American Citizens at the group’s convention next month in Washington.

Thursday’s proceedings took place amid the backdrop of the U.S. Supreme Court’s 4-4 decision on a case involving Mr. Obama’s deportation amnesty, which left in place a lower court’s ruling that blocked the implementation of the president’s executive action.

Mr. Sanders addressed the conference before the ruling came down, but issued a subsequent call for Senate Republicans to confirm a ninth justice to the court, which has one vacancy as a result of the death of Justice Antonin Scalia.

After his address to the group, Mr. Johnson said he agrees with Mr. Obama’s executive actions on immigration, but that he probably would want to reduce deportations even more as president.

Mrs. Clinton called the Supreme Court’s decision “unacceptable,” while Mr. Trump said it blocked “one of the most unconstitutional actions ever undertaken by a president.”

Despite Mrs. Clinton’s absence, Democrats are banking on Mr. Trump’s rhetoric about Latinos and immigration moving Hispanics toward the Democrats in the general election.

A super PAC supporting Mrs. Clinton’s campaign quickly sprang into action after the ruling, teaming up with the Latino Victory Fund to release a digital ad that quotes Mr. Trump as saying he would “immediately terminate” Mr. Obama’s executive actions on immigration.

Mr. Trump, meanwhile, said Mrs. Clinton’s immigration policies would hurt poor African-American and Hispanic workers “by giving away their jobs and federal resources to illegal immigrant labor.”

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