Democrats in Congress, increasingly concerned about how Israel is waging war in Gaza, are weighing whether to use their leverage over weapons sales to register objections to the civilian death toll and ratchet up pressure on President Biden to place conditions on American backing for the military offensive.

While the top Republicans on the congressional foreign affairs committees have signed off on a State Department plan to sell $18 billion worth of F-15 fighter jets to Israel, according to several people familiar with the consultation, the deal remains in limbo. That strongly suggests that the top two Democrats on the Senate Foreign Relations and House Foreign Affairs Committees have yet to sign off.

Spokesmen for the two — Senator Ben Cardin of Maryland and Representative Gregory W. Meeks of New York — declined to comment on the status of the deal, which would be one of the largest U.S. arms sales to Israel in years, and would also include munitions, training and other support. But other Democrats have said in recent days that Congress should be using its influence over weapons transfers to demand that Israel do a better job of protecting against civilian casualties in the conflict and allowing aid to reach civilians in Gaza.

An aide to Senator Chris Van Hollen, Democrat of Maryland, said on Wednesday that he was strongly considering several legislative options for doing so, including introducing a measure that would block weapons transfers. That would be an exceedingly long shot; it would take a supermajority in both chambers of Congress to overcome a veto by Mr. Biden, an almost impossible threshold given the strong bipartisan backing for Israel on Capitol Hill.

But lawmakers can use their oversight role to try to weigh in on the issue. Here is how it works.

The president must consult with Congress on major weapons sales.

Under the Arms and Export Control Act, the president must consult with Congress on large transactions that involve sending weapons of war to other countries.

If an order for military equipment reaches a certain monetary threshold — $25 million for close U.S. allies, including Israel — the president must formally notify Congress. The threshold is $100 million for defense articles or services and $300 million for design and construction services.

Fewer than 10 percent of all U.S. arms sales to foreign governments reach those levels, according to multiple people familiar with the consultation process, who were not authorized to comment publicly on it. This means that Congress reviews only the largest and most significant proposed deals.

After an informal notification, lawmakers can place a hold if they have objections.

Once the State Department has decided to move forward with a transfer, a draft of the deal is sent to the top members of the Senate and House foreign affairs committees for an informal review that is arguably the most important step in clearing any weapons sale.

The chairman and ranking member of both panels and their senior aides can raise any concerns or objections in private briefings with State Department officials, including technical questions about the capabilities of weapons being delivered, logistics for how they will be stored and who the end users will be.

Lawmakers can also register foreign policy concerns with the government in question, including on human rights and how the weapons will be deployed. The process can drag on if lawmakers are not satisfied with the answers. And if the concerns persist, a member can place a hold on the proposed transfer.

Sometimes the holds are temporary, but other times they can last months or years and ultimately sink a deal. They can be a great source of frustration for an administration hoping to push through a weapons deal quickly.

The administration can move forward without consent from Congress during the informal review period, but typically will move forward only if there are no more lingering concerns.

A formal notification usually means a done deal.

Once any congressional issues have been addressed, the State Department sends Congress a formal notification of the administration’s intent to continue with the deal.

The length of the review period varies by country; it is 15 days for sales to Israel. No deal can be finalized before the review period has concluded, but a formal notification usually means a deal is on the fast track for approval.

Still, during this period, any member of the House or Senate can file a resolution of disapproval to register objection to a deal.

Congress can move to block a weapons transfer.

To stop a weapons transfer at this stage, a resolution of disapproval would have to pass both the House and Senate and then overcome a certain veto by the president backing the deal. That would require a two-thirds majority vote in both chambers, something that has never happened.

The strong bipartisan backing for Israel in Congress makes it highly unlikely that things would reach this point; any disapproval resolution would be all but certain to fail. But the process could still lead to a public standoff between Democrats in Congress and the White House that Mr. Biden would certainly wish to avoid.

The president can bypass Congress.

The president has the authority to bypass the review period if he declares that an expedited emergency sale is in the “national security interest of the United States.” The administration is still required to notify Congress and provide details for invoking the emergency powers.

In 2019, the Trump administration used an emergency declaration to bypass the congressional notification process and push through a multibillion-dollar arms deal to Saudi Arabia, Jordan and the United Arab Emirates. The move angered both Democrats and Republicans who were critical of the Saudi-led coalition striking civilian targets in Yemen and angry about human rights abuses, including the killing of the journalist Jamal Khashoggi.

In that case, both chambers voted to block the deal but failed to override a veto by then-President Donald J. Trump.

Under the Biden administration, similar emergency powers have been used to fast-track aid packages to Ukraine and Israel. No resolution of disapproval has been introduced to block emergency use, but a number of Democrats expressed their frustration when Biden bypassed Congress twice in December to transfer more than $250 million in weapons to Israel. They warned Mr. Biden’s team against skirting congressional notification for future weapons transfers.

“Decisions of war, peace and diplomacy should be made through a process that is deliberate, transparent and consistent with our values,” Senator Tim Kaine of Virginia, one of the Democrats who objected to the earlier emergency declarations, said on Wednesday. “That means Congress and the American people must have full visibility over weapons we transfer to any other nation.”