Secretary of State John F. Kerry, right, speaks with Iranian Foreign Minister Mohammad Javad Zarif, ahead of nuclear talks in January. (Laurent Gillieron/Associated Press)

THE DEBATE in Congress over the most consequential U.S. diplomatic agreement in decades is headed for a weak and unsatisfactory ending. The Senate has ensured that the nuclear deal with Iran will not be overturned by Congress; while we supported that outcome, the preemption of a full debate through the use of a filibuster was disappointing. So, too, was the diversion by the Republican-led House into a far-fetched attempt to derail the accord with a claim that President Obama failed to meet a legal requirement that all of its details, including separate technical arrangements between Iran and United Nations inspectors, be disclosed. At best, the argument is a distraction: Congress cannot demand that the International Atomic Energy Agency alter its protocols to suit U.S. lawmakers.

The bottom line is that an agreement that will reshape U.S. policy in the Middle East, and could determine whether nuclear proliferation can be controlled in this century, stands to go forward without ratification or any other significant action by the legislative branch. That’s a particularly disturbing result in view of the bipartisan legislation on Iran passed earlier this year, which set up the review process that has now been curtailed or diverted.

Congress, however, still has an opportunity to shape how the nuclear deal is implemented. It can do so by taking up some of the proposals for supplementary legislation that are being advanced by both supporters and opponents of the agreement. Among others, Sen. Benjamin L. Cardin (D-Md.), an opponent, has drawn up a bill mandating follow-up steps, while Sen. Tim Kaine (D-Va.), a supporter, has been among those seeking to forge a bipartisan group like that which supported the review legislation. To his credit, Sen. Bob Corker (R-Tenn.), the chairman of the Foreign Relations Committee, has expressed interest.

The measures that could be included in a post-deal package start with a clarification of U.S. intent regarding Iran’s pursuit of a nuclear weapon. Congress can make clear that Mr. Obama or his successor will have support for immediate U.S. military action if an Iranian attempt to build a bomb is detected. One trigger could be verification that Tehran is producing highly enriched uranium; another would be the resumption of work on warhead designs and materials. Such a statement by Congress could not be binding, but it would tell Iran and U.S. allies in the region that the nuclear deal has not taken the U.S. military option off the table.

Other steps could be aimed at deterring Iran from using the billions it will gain from the lifting of sanctions to step up its support for Hezbollah, the Assad regime in Syria and other proxies. Tehran is claiming that the accord prevents the United States from reimposing sanctions in the future. But Congress can make clear that new sanctions can and will be adopted for non-nuclear offenses, such as weapons deliveries to Hezbollah or sponsorship of a terrorist attack on a U.S. target.

Legislation can also mandate new U.S. support for Israel. A 10-year security agreement, due to expire within three years, could be renewed and expanded; Congress could support the delivery to Israel of the Massive Ordnance Penetrator, a bomb developed to destroy an Iranian nuclear facility buried under a mountain.

The greatest risks of the nuclear accord are that Iran will seek a bomb in spite of the constraints it accepted and that it will escalate its attempt to establish hegemony over the Middle East by force. While Congress can’t now overturn the deal, it can pragmatically address both of these threats.