In his six years in the Senate, Cory Booker has progressed from a moderate who defended private equity to a leading progressive voice on issues like criminal justice reform and marijuana legalization. As he transitions to a national presidential campaign, which he announced Friday, the candidate has been focusing on some key issues that animate the left wing of the Democratic Party.

Criminal Justice

Mr. Booker has made criminal justice reform a cornerstone of his Senate tenure. He sought early on to craft bipartisan bills that would have changed penalties for nonviolent crimes and reduced prison sentences, though neither effort became law. Late last year, Congress passed a criminal justice reform act, signed by President Trump, that Mr. Booker had originally sponsored. But during his time as mayor, Newark’s police department faced accusations of brutality and intimidation, and the Department of Justice launched an investigation into the department; Mr. Booker said he “welcomed” the inquiry.

Health Care

In 2017, Mr. Booker announced his support for the Medicare for All Act drafted by Senator Bernie Sanders, and reiterated his support in an interview after he announced his candidacy Friday, saying “I signed up and am a big believer in Medicare for all.” But as a senator from New Jersey who had received millions in campaign contributions from the pharmaceutical industry, Mr. Booker had been reluctant to focus regulatory efforts on the industry, voting against a measure in 2017 that would have greatly reduced drug prices by allowing imports from Canada. He has since reversed course, announcing his support this year for Mr. Sanders’s bill to rein in prescription drug costs.

Green New Deal

Mr. Booker said Friday that “environmental justice” will be one of the three top policy issues of his campaign. He has recently signed on to endorse the Green New Deal, a progressive litmus test on the environment pushed by Representative Alexandria Ocasio-Cortez of New York, that pushes for investments in clean-energy infrastructure and policies to directly address climate change.