Sen. Charles E. Grassley (R-Iowa) recently said he was pushing ahead with a bill to limit President Trump’s authority to unilaterally impose tariffs. No matter what you think of Trump’s trade policies, Grassley’s bill is a great idea.

Anyone familiar with our Constitution should wonder why the president has such power in the first place. The Constitution clearly grants Congress the exclusive power to “lay and collect Taxes, Duties, Imposts, and Excises.” Indeed, it is the first power that the document grants Congress, ahead of the more famous powers to regulate commerce or declare war.

Case closed, it might seem.

Think again. Congress has been slowly giving away its powers over the past century, lodging them in independent agencies that write rules — and in the president himself. The Supreme Court has permitted this to happen under what is called nondelegation doctrine. In theory, this doctrine holds that Congress can only delegate its legislative powers when it provides an “intelligible principle” that guides and limits agency or executive authority. In practice, the court has not overturned a law delegating such power in more than 80 years.

Congress is the proper body to levy tariffs because of their political nature. Its members are directly responsible to the people through elections, and its debates are, ultimately, public. It is, therefore, well situated to both mediate disputes between competing interests and make someone responsible for the final decision. Grassley’s bill is, therefore, the right thing to do, both constitutionally and politically.

But there are complicating factors. The bill would need to determine the status of the tariffs that the president has already imposed under the delegated authority. If the bill immediately removed or eliminated all of those tariffs, it would certainly result in a presidential veto and likely provoke political backlash from Trump partisans.

China also muddles the bill. Grassley has previously said he generally supports the president’s decision to confront China over its trade practices, but it is Trump’s unilaterally imposed tariffs that brought about the confrontation to begin with. Addressing presidential tariff powers without formulating a clear approach to dealing with China would be a grave error.

One way to address both issues would be to limit which countries a president can declare a national security threat to the United States. Trump has imposed his tariffs as a result of a delegated authority to declare that some trading arrangements with other nations constitute such a threat. While many are coming around to this argument on China, they remain unconvinced that auto imports from Germany or imports from Canada or Mexico genuinely comprise a serious threat.

Grassley’s bill could delineate this difference by preventing a president from imposing such tariffs against imports from any country with which the United States has a treaty committing it to defend that nation. Such a limitation would exempt nations in NATO, as well as nations such as Japan, South Korea and Australia, from potentially crippling tariffs. But this would permit Trump’s tariffs against China or another global rival, such as Iran or Russia, that stands outside the orbit of U.S. allies.

The tariff bill should also be the start of a more general congressional effort to recapture its authority. Grassley is a co-sponsor of Sen. Rand Paul’s (R-Ky.) REINS Act, which would require congressional approval of any regulation promulgated by the executive branch that would create an annual economic impact of $100 million or more. Grassley can reassure advocates of limited and more efficient government by advancing this proposal in addition to his attempt to claw back authority of tariffs.

Trump has been right to talk about the harm that trade agreements have caused for millions of U.S. citizens. Grassley is nonetheless right that the president ought not to have the sole authority to abrogate or effectively amend such agreements. Hopefully his bill restores some of Congress’s constitutional power while also pointing a way forward to a successful resolution of our trade dispute with China.

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