Supporters of the Paris Agreement have expressed considerable angst over President Donald Trump’s decision to leave the climate agreement last week.

Regardless of the merits of that decision—although for the record, Heritage Foundation analysts recommended that the U.S. withdraw—Trump should not have had to make that call.

The Paris Agreement had all the hallmarks of a treaty and should have been submitted by President Barack Obama to the Senate for advice and consent, as required under Article II of the U.S. Constitution.

Instead, Obama signed the Paris Agreement as an executive agreement.

He was able to do so because of ambiguity over how international agreements are implemented in the United States.

There are different types of international agreements: treaties, executive agreements, and congressional-executive agreements. Precisely which agreements constitute treaties requiring Senate advice and consent in accordance with Article II of the Constitution, and which do not, is not always clear.

The uncertainty persists despite internal guidance adopted by the State Department, known as the Circular 175 procedure that lays out eight factors for determining whether an international agreement should be negotiated as a treaty or an “international agreement other than a treaty.”

While there is no statutory language setting clear policy in this area, there is an extensive history of how different subjects have been categorized. Moreover, there is record of deliberations from 1992 when the Senate gave its advice and consent to the U.N. Framework Convention on Climate Change.

Both indicate that the Paris Agreement should have been submitted to the Senate for advice and consent. As former Heritage research fellow Steven Groves wrote a year ago:

The Obama Administration’s unilateral treatment of the Paris Agreement is particularly disquieting for two reasons: (1) the agreement has all the hallmarks of a treaty that should be submitted to the Senate for its advice and consent under Article II, Section 2, of the U.S. Constitution; and (2) the agreement contains “targets and timetables” for emissions reductions and, as such, the Administration’s failure to submit the agreement to the Senate breaches a commitment made by the executive branch to the Senate in 1992 during the ratification process of the [U.N. Framework Convention on Climate Change]. …

In treating the Paris Agreement as a sole executive agreement, President Obama is using the fact that there is no statutory definition of what is and is not a ‘treaty.’ This strategy, however, ignores the fact that the State Department has an established process, known as the Circular 175 Procedure (C-175), to guide its decision to designate an international agreement.

Obama chose not to treat the Paris Agreement as a treaty for one overriding reason: He knew it would not receive support from the two-thirds of the Senate necessary for ratification.

In short, he circumvented proper procedures in an effort to secure his climate legacy.

As many have noted, Obama’s reliance on executive authority has made his legacy vulnerable to changes by the next president. Obama lost the gamble that his decisions would remain in place when Trump was elected.

Trump followed through on his campaign promise last week when he announced that the U.S. would withdraw from the Paris Agreement.

>>> 4 Reasons Trump Was Right to Pull Out of the Paris Agreement

Trump’s decision was based on a number of economic, political, and legal reasons. However, his apparent path of withdrawal—withdrawing from the Paris Agreement under the terms outlined in Article 28 (1) and (2) of the treaty—will not be complete until November 2020.

At the very least, the political and legal headaches anticipated by the White House will continue for the next three years. A faster path would be to exit the U.N. Framework Convention on Climate Change, which would take only one year as stated in Article 28 (3) of the Paris Agreement and Article 25 of the framework.

Trump should also give serious thought to supplementing his withdrawal from Paris with a declaration that the agreement is a treaty that Obama entered into in contravention of both past U.S. practice, and promises made to the Senate in 1992—and, therefore, is not binding on the U.S.

This may face legal challenge, but it would have two benefits.

First, arguing that the Paris Agreement is a treaty would make it more controversial for a future administration to reverse Trump’s withdrawal and, again, sign the U.S. up to the Paris Agreement as an executive agreement.

Second, it would provide a long-overdue defense of the legislative branch’s constitutional role in foreign policy, which has been eroding for decades.

>>> Read The Heritage Foundation’s full report, “The Paris Agreement Is a Treaty and Should Be Submitted to the Senate

The fact that more senators were not and are not outraged by Obama’s usurpation of their constitutional authority is extremely troubling.

Presidents should have to consult with Congress and secure broad consent—whether concurrent or pre-existing—before entering into agreements like Paris.

The authors of the U.S. Constitution required advice and consent of two-thirds of the Senate prior to treaty ratification.

They did so because they knew the dangers of entering into binding agreements with foreign powers without ensuring that the agreement has the support of a broad cross section of the American public and of the individual states.

Public officials on both sides of the aisle should be concerned over this matter. After all, senators often serve in office far longer than individual presidents and, therefore, overlap with presidents of the opposite party.

To address this issue over the long term, Congress should engage with the Trump administration to examine past practice on how international agreements have been treated historically.

Together, they should specify the exact issues or policy context that should mandate international agreements be submitted to the Senate as treaties under Article II. Congress should then codify this practice in law, both for treaties and other international agreements.