Reaching a comprehensive nuclear deal with Iran faces formidable challenges. As negotiators meet in Vienna this week to begin drafting a long-term deal that will limit Iran’s nuclear program, they do not need the U.S. Congress throwing additional roadblocks onto the path.

Members of Congress continue to make unreasonable demands about eliminating Iran’s uranium enrichment capacity, despite Washington having committed last November to a final deal with Iran that includes a “mutually defined enrichment program.”

Most recently, Rep. Doug Lamborn, R-Colo., offered an amendment to the fiscal 2015 National Defense Authorization Act that sets naïve and unrealistic expectations for the final deal, including ceasing enrichment.

While complete dismantlement of Iran’s nuclear program may have been have been ideal ten years ago, it is not feasible now, and it is necessary to guard against a nuclear-armed Iran. A final deal with stringent limits and intrusive monitoring and verification will accomplish that. What’s more, insisting on complete abandonment will only drive Tehran away from negotiations and an unrestrained enrichment program.

Some officials and policymakers are under the impression that Iran is required to give up its uranium enrichment activities because of U.N. Security Council resolutions. But this view is not supported by the facts. Dating back to 2006, Security Council resolutions required Iran to suspend its nuclear activities while a deal was reached that allayed the international communities concerns about possible work on nuclear weapons.

Building trust in the peaceful nature of Iran’s nuclear program will take time. But Tehran is taking the right steps to demonstrate that it is not seeking nuclear weapons and faithfully implementing the terms of the interim November deal, including limits on its uranium enrichment program and increased monitoring of its facilities.

This does not, of course, mean that the United States should accept an unlimited nuclear program in Iran. Given Tehran’s past covert nuclear activities, limits on enrichment are reasonable and necessary.

Iran should be restricted to producing uranium enriched to civilian power reactor grade. And the amount of uranium that Iran is allowed to enrich should be limited to a realistic assessment of its current needs — not the nuclear power program Tehran plans to build in the future.

In addition, Tehran must also allow international inspectors more extensive access to its nuclear facilities, visits on short notice, and must provide more timely information about future nuclear facilities.

Including these measures in a deal would ensure that Iran could not dash to build nuclear weapons before the international community would detect such activities and be able to block them.

Iran would not be the first country to overcome a history of mistrust regarding its nuclear intentions to pursue a peaceful nuclear program. Argentina has a uranium enrichment program for peaceful uses, despite having pursued a covert nuclear weapons program in the 1980s. South Africa still possesses a stockpile of uranium enriched to weapons levels, despite having built at least six crude nuclear weapons in the 1980s. Sweden conducted tests on nuclear weapons components after World War II. We no longer suspect these countries of using civilian nuclear capabilities to pursue weapons. Over time, with the right limits and verification, Iran can demonstrate that it too has left behind any intention of pursing nuclear weapons.

For the first time in over a decade, Iran has taken steps to roll back some of its most worrisome nuclear activities. And a long-term nuclear deal is within reach. What U.S. diplomats need from Congress is the time and support to negotiate a final agreement — not legislation that sabotages the prospects for a final deal to prevent a nuclear-armed Iran.

Kelsey Davenport is the non-proliferation analyst for the Arms Control Association, where she focuses on the nuclear programs in Iran and North Korea and nuclear security issues.