A Supreme Court case examining state deadlines for election ballots sent in the mail is just the tip of the spear in President Donald Trump’s war on mail-in voting.

Trump has never let go of his unfounded beliefs that voting by mail is rife with fraud, as he has called on Congress, the judiciary and state legislators to cut back on the practice. At his insistence, US Senate Republicans have devoted several days to debating an election overhaul bill that Trump wants to end most mail-in voting. But it is expected to die in Congress due to a Democratic filibuster.

Where Trump has seen some success on the issue is what the Supreme Court will scrutinize on Monday: the practice by some states of counting mail ballots that arrive at election offices after Election Day.

His Justice Department convinced at least one state to eliminate its mail ballot grace period by threatening a lawsuit, and other GOP-led states changed their deadlines proactively. On Monday, Trump’s top Supreme Court lawyer will argue in support of a challenge to Mississippi’s mail ballot deadline that was brought by the Republican National Committee.

The mail voting deadlines of more than a dozen states – including states like California, Texas and Alaska that will be pivotal in determining which party controls Congress – are now in jeopardy as the Supreme Court examines whether those policies run afoul of federal law.

The dispute is the latest example of Trump’s push to put the federal government’s heavy hands in the machinery of election administration – a task the Constitution largely delegates to the states. The president has suggested that he has the power to implement voter ID requirements on his own that are part of the election overhaul bill. He also demanded that GOP lawmakers add to that legislation a ban on no-excuse absentee voting.

A loss in the Supreme Court could further enrage a president who recently fumed at the justices for not overturning his 2020 electoral defeat. A decision would likely come before the end of June.

The legal challenge centers on statutes passed by Congress in the 19th and early 20th centuries that standardized federal elections for the Tuesday after the first Monday in November. Republicans argue those statutes preclude states from setting post-Election Day ballot receipt deadlines.

“Election-day receipt promotes election integrity and voter confidence as much today as it did when Congress passed that law,” the Trump administration told the Supreme Court in a legal brief.

Trump’s opponents counter that the federal Election Day statutes don’t go that far, and that states have flexibility in whether to embrace mail-in ballots and how to regulate them. They argue that if the Supreme Court were to accept the GOP’s logic, it would make it impermissible for states to offer mail-in voting or even early in-person voting.

“What Donald Trump wants, and has said that he wants, is Election Day voting only, in-person, only,” Marc Elias, a lawyer for voter groups that are defending the state laws, told reporters Monday. “So doing away with vote-by-mail is like the most convenient way for them to start, but it will not be where they end.”

An executive order Trump signed a year ago was his first attempt to rein in voting by mail, using multiple federal levers to pressure states to drop post-election grace periods for mail-in ballots. It instructed the withholding of federal election funding from those states, while also commanding the attorney general to “take all necessary action to enforce” the federal Election Day statutes.

A top attorney in the Justice Department’s Civil Rights Division suggested in court last April that the executive order could lead to criminal actions against states that refuse to move up their mail ballot deadlines.

Such criminal actions never materialized. But the attorney, Michael Gates, who has since left the department, used the threat of a civil lawsuit against at least one state to force it to change its law.

In early September, Gates emailed Ohio state officials a draft lawsuit that argued the federal Election Day statute prohibited the state’s acceptance of mail-in ballots up to four days after the election. He also offered a proposed legal agreement – known as a consent decree – that would have avoided litigation by having Ohio agree to have its deadline changed by court order instead.

According to communications obtained by CNN via a public records request, the state officials convinced Gates that Ohio lawmakers would be amenable to passing a bill to change the deadline instead, so that court action would not be necessary. Ohio Attorney General Dave Yost and Secretary of State Frank LaRose, both Republicans, advocated in favor of the bill last year by pointing to the threat of litigation.

Yet when GOP Gov. Mike DeWine signed the bill, he said he was doing so “reluctantly,” according to the Statehouse News Bureau.

Another three states – Kansas, North Dakota and Utah – eliminated their grace periods in the past year. North Dakota’s deputy secretary of state, Sandy McMerty, told CNN that the state was responding to the Trump executive order and to the legal fight now at the Supreme Court.

Courts have since blocked much of executive order and the bulk of its provisions seeking to pressure states on their mail ballot deadlines.

The Supreme Court could hand Trump the win he was not able to achieve through unliteral action, though legal experts are skeptical of the Republican arguments for why the state policies are unlawful.

A ruling from a conservative appeals court in New Orleans that sided with the GOP was likely the reason that Supreme Court felt the need to take up the case, according to Rick Hasen, an election law professor at the University of California, Los Angeles.

The case started as a 2024 Republican National Committee lawsuit against Mississippi’s law giving mail ballots five days to arrive after Election Day. Mississippi severely limits who can vote absentee, so the RNC’s choice to challenge the policy there was seen as a strategic move to get the case before the 5th US Circuit Court of Appeals.

Thirteen other states, as well as the District of Columbia, have post-Election Day mail ballot receipt deadlines, and 15 more have extended deadlines for certain ballots submitted by military families on deployment and US citizens overseas, according to the National Conference of State Legislatures.

The Trump administration is asking the Supreme Court not to eliminate the grace periods for military ballots.

The 5th Circuit majority embraced the arguments by Republicans that Congress – when it set a federal Election Day – meant for that day to be the final day for ballots to be received by election officials. But other judges who looked at the question have disagreed, saying that the laws’ text includes no such receipt-deadline for mail-in ballots and only require that individual voters make their choice by Election Day.

The Justice Department, under President Joe Biden, initially supported Mississippi. But once the case reached the Supreme Court, and after Trump had returned to the White House, the Justice Department not only switched sides, but also sought the opportunity to make oral arguments at Monday’s hearing.

It was once rare for the Justice Department to insert itself in a case it was not a party to when the justices hadn’t asked for its input. But Trump’s solicitor general, John Sauer, has been piping in on high-profile cases on a more frequent basis. Still, Sauer’s decision to argue the ballot deadline case on Monday, rather than assign it to one of his deputies, shows its significance to the administration.

Several Republican state attorneys general, GOP members of Congress and conservative election integrity groups have also lined up behind the Republicans’ arguments. By contrast, Trump’s demand that Congress add a national ban on no-excuse absentee voting to legislation focused on voter ID and citizenship verification prompted debate among Senate Republicans, many of whom who represent states with expansive mail-in voting policies.

Jason Snead, executive director of the Honest Election Project, which filed a friend-of-the-court brief supporting the RNC, said that requiring Election Day mail-in ballot deadlines is a “very popular position with the voters,” and “a best practice as far as election administration is concerned.”

“I think everybody understands that the ballot is really considered a vote when it’s received by an election official,” Snead told CNN. “That’s a bright line rule, and that brings clarity to the process.”

Despite the momentum Trump has seen in his broadsides against mail-in ballot grace periods, Hasen predicted a unanimous or near-unanimous ruling from Supreme Court upholding the state policies, which in turn will provoke a “ballistic” response from Trump.

“The backup position is always, if the courts or Congress can’t make these changes, the election is full of fraud,” Hasen told CNN, adding that if Democrats win in November’s midterms, it’s “a convenient backup.”