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.

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.

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 2017that 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.

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.

From his time as mayor of Newark, Mr. Booker has been a strong champion of charter schools, a stance that runs counter to the progressive Democratic base. And it’s a position that he continues to hold. He said in a 2018 interview that “my loyalty is to a free public school, high-quality public school education,’’ adding that this includes charter schools.

Education reform also gave Mr. Booker his most high-profile moment as mayor of Newark, when he went on The Oprah Winfrey Show with Gov. Chris Christie to announce that Mark Zuckerberg would be donating $100 million to turn around Newark’s ailing school system. Though Mr. Booker defends the reforms made with the money, many critics have said it was not put to good use, and the city’s system continued to teeter.

Mr. Booker has long been a proponent of many core Democratic issues, such as protecting a woman’s right to choose and favoring same-sex marriage. He presided over New Jersey’s first same-sex marriage ceremonies as a senator-elect in 2013. From his position on the Senate Judiciary Committee, he recently challenged Justice Brett M. Kavanaugh and Secretary of State Mike Pompeo over their views on gay rights.

In 2017, Mr. Booker introduced legislation that would have legalized marijuana and expunged federal marijuana convictions from criminal records. While the bill had no hope of passing the Republican-held senate, Mr. Booker reiterated his support for legalizing marijuana after announcing his candidacy, telling a radio show that he was for “changing our drug laws, ending prohibition against marijuana.”