As Joseph R. Biden Jr. makes his third run for the White House, he is being pressed to answer for his role in legislation that criminal justice experts say helped lay the groundwork for the mass incarceration that has devastated America’s black communities.

Among the most significant were: the Comprehensive Crime Control Act of 1984, which established mandatory minimum sentences for drug offenses; the 1986 Anti-Drug Abuse Act, which imposed harsher sentences for possession of crack than for possession of powder cocaine; and the Violent Crime Control and Law Enforcement Act of 1994, which was essentially a catchall tough-on-crime bill.

During the 1980s and 1990s, when Mr. Biden was a senator from Delaware, he and other leaders of the powerful Senate Judiciary Committee helped fashion a string of bills that overhauled the country’s crime laws.

Mr. Biden long defended the 1994 crime bill, saying as recently as 2016 that it “restored American cities” and that he was not ashamed of it.

He apologized in January for portions of his anti-crime legislation, but he has largely tried to play down his involvement, saying in April that he “got stuck with” shepherding the bills because he was chairman of the Senate Judiciary Committee.

His campaign has maintained that Mr. Biden has “fought to defeat systemic racism and unacceptable racial disparities for his entire career” and that he “believes that too many people of color are in jail in this country.” Current statistics from the United States Bureau of Prisons show that African-Americans, who make up roughly 12 percent of the American population, account for 37.5 percent of the federal prison population.