Congress must vote during the lame duck on a measure to extend the existing regime of U.S. sanctions on Iran without upsetting the delicate balance on the nuclear pact. (AP Photo/Fars News Agency, Omid Vahabzadeh)

When Congress returns to Washington later this month to tackle a budget impasse and a massive defense policy bill, there will be one more contentious item on its agenda: extending sanctions on Iran.

At the end of the year, the Iran Sanctions Act (ISA) expires — and with it, the regime of existing U.S. sanctions lawmakers say are essential to ensure Washington can “snap back” punitive measures against Tehran should Iranian leaders violate the terms of the nuclear deal that went into effect earlier this year.

But if lawmakers go too far in their bid to renew and possibly stiffen the sanctions, the White House fears Tehran could interpret it as a U.S. violation of the deal — and take that as a cue to fire up their nuclear reactors again.

Few issues of foreign policy have divided Congress as bitterly as the Iran deal.

Many Democrats championed it as President Obama’s flagship diplomatic achievement, while Republicans blasted the accord as a foolhardy mistake that would only empower Iran and make it a bigger threat in the Middle East, particularly to Israel.

Lawmakers today are similarly divided over the best approach to renewing the energy, banking and defense industry restrictions on Iran’s nuclear and missile activities.

For many, the bill to renew the sanctions for 10 years is the best opportunity to slap new penalties on Tehran for a spate of ballistic missile tests it has conducted since the nuclear deal was struck. Democrats and Republicans have called for a response to the tests, arguing they violate the spirit, if not the letter, of the nuclear deal that eased sanctions on Iran in exchange for a cessation of its nuclear program.

Others see the legislation as a chance to clamp down on the president’s ability to strike future financial settlements with Tehran — a cause Republicans seized upon after revelations that the White House had timed $1.7 billion in cash settlements to Iran to coincide with the release of American prisoners, which the GOP calls “ransom.”

And a few more Republicans lawmakers don’t want to see a renewal of Iran sanctions move ahead without also cutting a hefty check to Iran’s archenemy Israel, even if that flies in the face of an aid deal the Obama administration and Israel struck just a few weeks ago.

The discord is concentrated mainly in the Senate, where there are three competing proposals jockeying for a floor vote during the lame duck. In the House, the normally pugnacious GOP majority — which has passed a litany of measures aimed at limiting the Iran deal and U.S. engagement with Tehran — is opting for a bipartisan approach. The House is planning to vote on a clean 10-year renewal of the existing sanctions, according to a GOP congressional aide

The Iran Sanctions Act “should remain in place until the regime stops exporting terror and threatening us and our allies with deadly weapons,” said House Foreign Affairs Committee Chairman Edward R. Royce (R-Calif.), who is sponsoring the measure. “That’s why I’ll be introducing a bipartisan, long-term extension of these important sanctions.”

But it is unclear if the Senate will follow the House’s lead — or if the White House is even ready to accept a bipartisan renewal of existing sanctions.

The effectiveness of the Iran deal relies on Iran upholding its promise to suspend its nuclear program and on the other parties to the agreement — including the United States — not creating any new nuclear-related sanctions against Tehran while the deal is in effect.

Some advocates of the deal have lobbied against renewing the sanctions out of concern that Iranian leaders might interpret the move as the U.S. violating its obligations under the nuclear pact. At the very least, they argue, a 10-year extension — which would extend beyond the date when the nuclear pact requires the U.N. to formally lift sanctions in 2023 — would exacerbate tensions during a sensitive time for the deal.

The White House is not worried that renewing the sanctions will violate the nuclear deal, but has argued it has the authority to impose penalties on Iran for malign activities even if the current law expires. The White House has not shot down the idea it could veto an extension of sanctions.

“I won’t prejudge at this point about whether the President would sign that bill, but I would just make the point that the kind of authority that Congress is saying the executive branch should have to confront Iran is the kind of executive authority that we already have,” White House spokesman Josh Earnest told reporters last week.

Congress isn’t buying that.

Lawmakers from both parties have expressed deep frustration at the pace and scope with which the Obama administration sanctioned Iranian individuals and entities over frequent ballistic missile tests conducted during the last year. Iranian leaders argue those tests are their right under the deal, but U.S. lawmakers warn they could have no purpose but to ready Iran’s capability to deliver a nuclear weapon in the future.

Such concerns drove Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) and others to push for a sanctions renewal bill that would supplement existing Iran sanctions with new mandatory punishments for those involved with Iran’s ballistic missile program, cyberthreats and espionage, and the country’s Revolutionary Guard Corps. The bill would also prohibit Iran’s financial institutions from conducting dollar-based transactions with banks in third-party countries.

Corker’s approach has tacit support from GOP Senate leadership, but its road to a floor vote is being challenged from both the right and left.

Republican Lindsey O. Graham (S.C.) threw a wrench into the mix this fall, when he threatened a counterproposal that would tie the extension of sanctions to approving an extra $1.5 billion in aid for Israel. Graham’s proposal was designed to challenge a 10-year memorandum of understanding the Obama administration struck with Israel, which guarantees unprecedented levels of military aid but stipulates that Israel cannot take more money from Congress. Still, aid for Israel is politically popular with both parties and Graham all but dared fellow lawmakers to vote against his proposal, should it be considered on the floor.

Meanwhile, most influential Senate Democrats have banded around a simple, no-frills reauthorization that Corker’s usual negotiating partner, Senate Foreign Relations Committee top Democrat Benjamin L. Cardin (Md.), proposed only days after the chairman introduced his bill. Their measure is most akin to the House’s forthcoming Iran sanctions bill that members are expected to vote on later this month.

A successful House vote on that bill may create enough momentum to carry a simple extension of the existing sanctions all the way through Congress and onto the president’s desk.