During last night's Democratic presidential debate, former Vice President Joe Biden admonished Sen. Kamala Harris (D–Calif.) for promising to impose new gun controls by executive fiat if Congress fails to pass the laws she thinks it should. That gave Harris a perfect opportunity to explain how her 100-day plan for gun control can be reconciled with constitutional restrictions on presidential power. The former prosecutor not only conspicuously failed to do so but literally laughed at the question.

The senator's campaign website promises that "if Congress fails to send comprehensive gun safety legislation to Harris' desk within her first 100 days as president—including universal background checks, an assault weapons ban, and the repeal of the NRA's corporate gun manufacturer and dealer immunity bill—she will take executive action to keep our kids and communities safe." Biden interprets that pledge as a promise to ban "assault weapons" without new legislation, something the president clearly does not have the authority to do.

Harris' plan for unilateral action on "assault weapons" is actually more modest than Biden implies. She says she would "ban AR-15-style assault weapons from being imported into the United States," noting that the Gun Control Act "empowers the executive branch to prohibit the importation of guns not 'suitable for or readily adaptable to sporting purposes.'" As Harris points out, "both Democratic and Republican presidents," including George H.W. Bush in 1989, have used that provision to block importation of "assault weapons." But two other parts of Harris' gun control plan do not seem to have any statutory basis.

Harris says she would "close the 'boyfriend loophole' to prevent dating partners convicted of domestic violence from purchasing guns." Under current law, people convicted of misdemeanors involving "domestic violence" are barred from possessing firearms. But crimes against dating partners count as "domestic violence" only if the perpetrator has lived with the victim or produced a child with him or her. Harris seems to think she can eliminate those requirements without new congressional action, but it's hard to see how. Congress has defined "misdemeanor crime of domestic violence," and only Congress can change the definition.

Harris also thinks the president can "mandate near-universal background checks by requiring anyone who sells five or more guns per year to run a background check on all gun sales." Since only federally licensed dealers are legally required to run background checks, such a rule would require dramatically expanding that category.

The problem is that federal law defines a gun dealer as someone who is "engaged in the business of selling firearms," which in turn is defined as "devot[ing] time, attention, and labor to dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms." The statutory definition explicitly excludes "a person who makes occasional sales, exchanges, or purchases of firearms for the enhancement of a personal collection or for a hobby, or who sells all or part of his personal collection of firearms." Under Harris' plan, a hobbyist or collector who sold more than four guns in a single year would be required to obtain a federal license and conduct background checks, which is plainly inconsistent with current law.

Instead of explaining the legal basis for the "executive action" she has in mind, Harris made a weak joke: "Hey, Joe, instead of saying, 'No, we can't,' let's say, 'Yes, we can.'" Then she launched into a description of the casualties from mass shootings, adding, "The idea that we would wait for this Congress, which has just done nothing, to act, is just—it is overlooking the fact that every day in America, our babies are going to school to have drills, elementary, middle and high school students, where they are learning about how they have to hide in a closet or crouch in a corner if there is a mass shooter roaming the hallways of their school."

That is not an argument in favor of any particular gun control policy, let alone an argument for the president's authority to impose it unilaterally. "Let's be constitutional," Biden said. "We've got a Constitution." To which Harris replied, in effect, "Constitution, schmonstitution. Why should that get in the way of my agenda?" Even voters who tend to agree with Harris about gun control should be troubled by her blithe dismissal of the legal limits on the powers she would exercise as president.