WASHINGTON — 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 expires — and with it, the regime of existing US 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 US 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 session. In the House, the normally pugnacious GOP majority — which has passed a litany of measures aimed at limiting the Iran deal and US 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, a California Republican 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 US 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 UN 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 said 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 US lawmakers warn they could have no purpose but to ready Iran’s capability to deliver a nuclear weapon in the future.