The Republican-controlled Congress did not pass any gun control legislation then, but Mr. O’Rourke continues to support similar policies, including universal background checks, magazine size limits and restrictions on some semiautomatic weapons.

He also opposes concealed carry reciprocity, which would make concealed carry permits granted in any state valid nationwide, forcing states with strict licensing requirements to recognize permits from states with looser standards. It is one of the National Rifle Association’s legislative priorities.

Health care

While Mr. O’Rourke supports universal health care — increasingly a litmus-test position for Democratic candidates — he hasn’t committed to a specific way to get there. During his Senate campaign, he suggested that universal health care could take the form of a single-payer system or “a dual system,” in which a government-run program would coexist with private insurance.

He has given conflicting messages on the most prominent proposal, “Medicare for all.”

In 2017, during his time in the House, he said unequivocally, “A single-payer ‘Medicare for all’ program is the best way to ensure all Americans get the health care they need.” But during his Senate campaign, he carefully avoided those words, calling instead for “universal, guaranteed, high-quality health care for all.”

He does support expanding Medicaid under the Affordable Care Act and has urged Texas to do so. Other points in his 2018 platform included allowing the government to negotiate prescription drug prices with manufacturers and creating incentives for insurers to participate in the Affordable Care Act’s exchanges.

Climate change

When it comes to proposals for mitigating climate change, opponents frequently cite the potential costs of those plans. But Mr. O’Rourke has emphasized the financial impact of climate change itself. Billions of dollars that are spent on steps like sea walls, which would be critical if climate change continues unchecked, is money not spent on other priorities, he noted at a town-hall event with Columbia University students last month.

At that event, Mr. O’Rourke spoke positively about the Green New Deal, calling it technically and logistically doable. As for its political prospects, “I don’t know,” he said. “That’s up to all of us, right?”