Twenty Democratic U.S. senators on Wednesday expressed support for a potential Israeli-Saudi normalization deal but laid out concerns about any U.S. security guarantees or nuclear assistance to Riyadh.

In a letter to President Joe Biden, the senators underscored resistance the White House could face from Congress if the administration brokers a landmark agreement opening diplomatic ties between long-time foes Israel and Saudi Arabia in return for Washington meeting Riyadh’s demands.

Negotiations have been advancing, but U.S. officials caution that much work remains.

Among the suggestions of Biden's fellow Democrats is that any agreement include "meaningful" provisions to preserve the option of a two-state solution to the Israeli-Palestinian conflict. Israel's far-right government is expected to resist any big concessions to the Palestinians.

“Peace between Israel and its neighbors has been a longstanding goal of U.S. foreign policy, and we are maintaining an open mind about any agreement that would potentially deepen the political, cultural and economic ties between Saudi Arabia and Israel,” the senators wrote.

But they cited misgivings about what the Saudis want.

Saudi Arabia is determined to secure a military pact requiring the United States to defend the kingdom in return for opening ties with Israel and will not hold up a deal even if Israel does not offer major concessions to Palestinians in their bid for statehood, three regional sources familiar with the talks have told Reuters.

“A high degree of proof would be required to show that a binding defense treaty with Saudi Arabia – an authoritarian regime which regularly undermines U.S. interests in the region, has a deeply concerning human rights record, and has pursued an aggressive and reckless foreign policy agenda – aligns with U.S. interests," the letter stated.

Both Democrats and Republicans have previously denounced Riyadh for intervention in Yemen, moves to prop up oil prices and its role in the 2018 killing of journalist Jamal Khashoggi.

The letter also stated concerns about the Saudis' request for civilian nuclear help and access to advanced weaponry.

It said the U.S. must hold the Saudis to the “gold standard” of Section 123 of the U.S. Atomic Energy Act, establishing a framework for peaceful nuclear cooperation.

The White House did not immediately respond to a request for comment.

Senators leading the letter effort included Middle East subcommittee Chair Chris Murphy, No. 2 Democrat Dick Durbin, Chris Van Hollen and Peter Welch.