Congressman Wiley Nickel, Democrat of North Carolina, is urging his party to “appoint a shadow cabinet to fight back against the worst excesses of a second Trump administration.” He’s borrowing the idea from the Britain’s parliamentary system, but it would be a poor fit for our republic, eroding the will of the people which invests executive power in only one president at a time.

“Across the Atlantic,” Mr. Nickel wrote in a Washington Post op-ed on Monday, “the British have something we don’t: a team from the opposition that mirrors the government’s cabinet members. They watch the cabinet closely, publicly challenging, scrutinizing, and offering new ideas.” That these ideas were rejected in the election escapes mention.

A prime minister isn’t chosen in a nationwide vote, and parliamentary systems have multiple parties rather than America’s two. The U.K. prime ministers sometimes form governments — including a cabinet — in coalition with smaller parties and serve only so long as they have support. In 2022, Prime Minister Truss stepped down after just fifty days. It was the shortest tenure in British history.

The American system is different by design. Presidents are elected to four-year terms, exercising co-equal power with the legislative and judicial branches. Their cabinets, while not laid out in the Constitution, perform a function it authorizes while a shadow cabinet would be created out of whole cloth to undermine the president, who is the sole individual vested with the executive power.

To ensure the nation spoke with a unified voice abroad, the Constitutional Convention empowered the Senate to offer “advice and consent” on foreign policy and appointments but didn’t lay out a mechanism for doing so. The job of putting this concept into action, as with so many things, fell to President Washington.

On August 22, 1789, four months after taking the oath of office, Washington met with senators in person, asking for their input on treaties with the Creek and Cherokee nations. The author of “The Cabinet: George Washington and the Creation of an American Institution,” Lindsay Chervinsky, laid out the difficulty that led the first president to create the cabinet.

Washington, seeking the Senate’s input, “carefully crafted” a statement of his aims in his proposed negotiations. When his vice president, John Adams, read it to the senators, Ms. Chervinsky writes that he was met with “awkward silence.” Senator William Maclay recorded that “his colleagues were so intimidated by” the great general “that they cowered in shameful silence.”

Maclay suggested that committees break off to discuss Washington’s remarks. The president, feeling he’d wasted his time, “lost his temper, stood up, and shouted, ‘This defeats every purpose of my coming here!’” Heading to his carriage, he “muttered under his breath that he would never return for advice.” He did go back two days later, having promised to do so, but never again.

All American presidents since have followed this precedent in letting their cabinet secretaries — based on Washington’s counsels of war during the Revolution — fulfill the Senate’s role. The first cabinet had four secretaries: State, Treasury, War, and Attorney General. The number has since grown to fifteen.

Cabinet secretaries are nominated by the president and confirmed by the Senate. The shadow body Mr. Nickel proposes wouldn’t face such scrutiny. Its members would be partisan appointees tasked with siphoning off the power the president has earned. Much like Democrats giving Speaker Pelosi the title of “speaker emerita” in 2022, a shadow cabinet would claim an unearned mandate.

In Ms. Pelosi’s case, her invented title doesn’t just infringe on the power of Speaker Johnson, chosen to lead by the House majority. It also undermines her own party’s minority leader, Congressman Hakeem Jeffries. His predecessor remaining in office even after losing the majority makes it difficult for him to chart a new direction for the Democratic caucus as he is no longer its sole voice.

“We can’t let,” Mr. Nickel wrote, “Donald Trump’s radical agenda go unanswered or unopposed.” Of course, no losing party is expected to end their opposition or silence their voices. The American people, though, saw Trump’s plans and judged them preferable to the Democratic alternative. The existing mechanisms for resisting them work just as well now as they have since Washington’s day.

Mr. Nickel writes that the Democrats “failed to convince the American people we have better ideas” in the election. They did, and they’ll have another chance in four years. Setting up a shadow cabinet in the meantime will only undermine national unity, violating the spirit if not the letter of the Constitution. America is not the U.K. It has no monarch — and only one president, one House speaker, and one cabinet at a time.