The bill was approved by the House but faces long odds of being passed by the Senate.

President Trump on Sunday said he would not sign any new legislation into law until Congress passes the SAVE Act, a Republican-led bill that would dramatically overhaul elections nationwide and potentially disenfranchise millions of voters.

“It must be done immediately,” he wrote on Truth Social. “It supersedes everything else. MUST GO TO THE FRONT OF THE LINE.”

The legislation, which Trump has urged Congress to pass for years, was approved by the House of Representatives in a near party-line vote last month, but it has stalled in the Senate. Democrats have vowed to block it from moving forward, and Republican leaders in the chamber have so far balked at calls to change procedural rules to get around a Democratic filibuster.

In his post on Sunday, Trump also demanded that additional provisions unrelated to voting be added to the bill, including new restrictions on medical care for transgender minors and a ban on transgender participation in sports.

Republicans say the election elements of the bill are needed to defend American democracy, often echoing Trump’s false claims about “stolen” elections.

“It’s essential to maintaining our constitutional republic, and everybody in this country seems to understand and agree to that,” GOP House Speaker Mike Johnson said after last month’s vote.

Democrats argue that the bill is purely designed to make it harder for voters, particularly those who tend to vote against Republicans, to cast their ballots.

“The SAVE Act is not an election security bill — it is a voter suppression bill, full stop,” the Congressional Black Caucus, a group whose members are all Democrats, said in a statement.

On Monday evening, Trump told a room full of Republican lawmakers that passing the SAVE Act would “guarantee” a GOP victory in November’s midterms, adding that the party would face “big trouble” without it.

What would the SAVE Act do?

Right now, rules around voter registration, the types of ID people need to show when they go to the polls and things like mail-in voting vary dramatically from state to state. The SAVE Act, which is officially named the SAVE America Act, comes well short of Trump’s ambition to “nationalize” American elections, but it would impose strict new rules for the whole country. These are some of its most important provisions:

Proof of citizenship for voter registration: The SAVE Act would require anyone registering to vote anywhere in the U.S. to show “documentary proof of United States citizenship.” Under the provisions in the bill, things like a driver’s license, Real ID or Social Security number would not be enough. Prospective voters would need to provide a valid U.S. passport, military ID, tribal ID or birth certificate.

These requirements would apply to new voters and to anyone who was reregistering, either because they moved to a new state or changed their name. It would create additional hurdles for women who take their partner’s name after getting married, who would need to provide extra documentation to explain why their current name doesn’t match what’s listed on their birth certificate or passport.

Only U.S. citizens are eligible to vote in most U.S. elections. States already require voters to certify, under the threat of legal penalties, that they are citizens when they register. States also audit their voter rolls regularly to purge the names of anyone who should not be listed. Though Trump and many Republicans have claimed that noncitizen voting is rampant in American elections, researchers have found a “shockingly small number” of actual documented incidents — far too few to impact the outcome of even small local elections.

Only eight states currently require documentary proof of citizenship for voter registration. Expanding these standards nationwide would prevent 21 million eligible voters from being able to vote, according to an estimate by the Brennan Center for Justice. Only half of all Americans have a passport. Millions more either don’t have or can’t readily access their birth certificates.

Nationwide voter ID: The SAVE Act would require all voters to present a valid photo ID when they cast their ballots. Though a majority of states have some sort of voter ID law in place already, most of those laws are far less stringent than the standard the SAVE Act would create. For example, a number of states accept non-photo IDs or have procedures that allow voters without ID to still cast their ballots. Only 10 states currently have the kind of strict voter ID laws in place that the SAVE Act would impose on the whole country.

Though a strong majority of Americans support voter ID laws, many experts say the rules do little to prevent fraud and instead disenfranchise otherwise eligible voters. Like non-citizen voting, cases of voter impersonation are exceedingly rare. There have been just 34 documented cases of someone falsifying their identity at the polls out of the billions of individual votes cast in the United States over the past 40-plus years, according to a database compiled by the conservative think tank Heritage Foundation.

Research on how many people voter ID laws prevent from voting is mixed, but most studies have found that it does have at least some impact on turnout. Elderly voters and people of color are disproportionately likely to lack the kind of photo ID they’d need to cast their ballots under the standards the SAVE Act would create.

New limits on mail-in voting: The SAVE Act would not override state mail-in voting rules — or fulfill Trump’s wish to ban the practice altogether. But it would make voting by mail harder. That’s because the new proof of citizenship rules would also apply to voters who register to vote by mail. Anyone registering for an absentee ballot, with some limited exceptions, would still have to go in person to a local elections office to present their passport or birth certificate for their registration to be valid.

Other provisions: The SAVE Act would also impose strict new requirements for how often and how thoroughly states would be required to audit their voter rolls for noncitizens, create criminal penalties for elections officials who allow someone who fails to show proof of citizenship to register and give regular citizens the power to sue elections officials who they believe violated the provisions of the law.

Will the SAVE Act become law?

At the moment, the prospects of the SAVE Act making it through the Senate seem dim. Republicans would need at least seven Democrats to vote in favor of the bill to overcome the filibuster, but the party is uniformly in opposition to the legislation — which their leader in the Senate, Chuck Schumer, has called “Jim Crow 2.0.”

Some hard-line Republicans in the House tried to create pressure to pass the SAVE Act by attempting to have it added to the crucial funding bill that ended the brief partial government shutdown in early February, but that effort came up short. They have also been calling on the top Republican in the Senate, John Thune, to amend filibuster rules to allow the SAVE Act to pass with a simple majority. Thune would need the backing of almost every GOP senator to do that, which he is unlikely to get.

“There aren’t anywhere close to the votes — not even close — to nuking the filibuster,” he said last month.

There’s no guarantee that the SAVE Act would pass even in a majority vote. Alaska Republican Sen. Lisa Murkowski has said that she opposes the bill, which she described as “federal overreach” into states’ power to run their own elections. Some other GOP senators, including Susan Collins of Maine and Mitch McConnell of Kentucky, have also expressed their misgivings about the legislation.

Trump’s leverage to force the Senate’s hand is also limited. Under federal law, any bill that sits idle on a president’s desk for 10 days automatically becomes law as long as Congress remains in session. Trump does have the power to veto bills, but lawmakers could override his veto with a two-thirds vote in both chambers.

Congress also has some major priorities that members of Trump’s own party may be reluctant to put on hold, including ending the ongoing shutdown affecting the Department of Homeland Security and confirming his new pick to lead the department, as well as potentially approving additional funding for the war with Iran.

On Monday, there were some signs that Trump’s firm stance against signing any other bills may have softened slightly. NBC News reported that the White House would make an exception for a bill to reopen DHS. Thune also told Punchbowl News on Monday that he was hopeful Trump might show flexibility on other issues as well.

“If we can get some stuff done [in the Senate], I hope we can get some things signed into law,” he said. “But that's a question I think probably for him.”