Allies of President Trump are alleging that the Maryland Democrat, Senator Van Hollen, is in violation of the Logan Act which bars citizens from conducting foreign policy. The law is central to Washington’s longest-running bit of political theater, one whose plot is about to take a twist.

Last week, Mr. Van Hollen traveled to El Salvador and held a photo-op with their national, Kilmar Abrego Garcia, who was deported while living in Maryland illegally. On Monday, four House members — Congressmen Maxwell Frost and Roboert Garcia, plus Maxine Dexter and Yassamin Anarsi — arrived at San Salvador to lobby for the alleged MS-13 gang member as well. 

The Trump Administration called deporting Mr. Abrego Garcia “a mistake,” but pleads impotence over Supreme Court order to “facilitate” his return from a country beyond their jurisdiction. Democrats dismiss this, turning the detainee into a cause célèbre.

President Nayyib Bukele of El Salvador, an ally of Mr. Trump, is playing his part with flair. He tweeted on Thursday that Mr. Abrego Garcia was “sipping margaritas” with Mr. Van Hollen in “tropical paradise.” On Sunday’s “Meet the Press,” the senator alleged that the Central American leader had played games by trying to choose a favorable backdrop.

Mr. Bukele proposed Mr. Abrego Garcia be met poolside, Mr. Van Hollen said, but he insisted on a restaurant. “Fake margaritas,” the senator said, were served as props. “All of that was a setup,” which he dubbed “margarita-gate.” 

With the action shifting back to Washington, the Logan Act is emerging as a “MacGuffin,” the plot device named by an English screenwriter, Angus MacPhail. “Brewer’s Dictionary of Twentieth-Century Phrase and Fable” writes that he chose “guff” to indicate “trivial or worthless.” 

The Logan Act is named for a Pennsylvanian, Senator George Logan, who — during the Quasi War with France in 1798 — parleyed with Paris behind President John Adams’s back. The law’s substance, like any MacGuffin, has never much mattered. 

On Wednesday, Mr. Trump’s friend, Roger Stone, asked why Mr. Van Hollen hadn’t been arrested under Logan. “It’s illegal,” he wrote, “to conduct your own foreign policy.” The syndicated radio host, Vince Coglianese, read the law for his audience and asked if they thought the senator was in violation. 

Others have pressed Attorney General Bondi to investigate. Logan will add weight to GOP criticism while allowing Democrats like the House delegation to strike a defiant posture against Mr. Trump on pilgrimages to San Salvador.

Congressman Jamie Raskin got in on the act with a veiled jab at Mr. Bukele. Any foreign leader, the Maryland Democrat said, who “facilitated authoritarianism” by working with Mr. Trump would pay a price when Democrats “come back to power.” This could earn him a part in the Logan show, too.

Just don’t expect any of the actors on our political stage to pull back the curtain, exposing that only two indictments have been brought under Logan in 226 years. The first prosecuted was a Kansas farmer, Francis Flournoy, in 1803.

Flournoy ran afoul of Logan by advocating for the then-westernmost states to secede and join French Louisiana. In 1853, a sea captain, Jonas Levy, was tried for lobbying Mexico’s president to reject a railroad treaty President Filmore favored. Neither man was convicted. 

For over 170 years since Levy sailed into history, Logan was more a rhetorical cudgel to be swung than a statute to be obeyed. That changed in Mr. Trump’s first term, when his national security advisor, General Michael Flynn, was accused of violations, furthering the notion that Mr. Trump had colluded with Moscow.

At issue was Flynn’s contact with the Russian ambassador, Sergei Kislyak, during Mr. Trump’s first transition. While the then-FBI director, James Comey, said that the communications were “legit,” an investigation was launched under the guise of enforcing Logan.

In a 2017 deal with the government, Flynn pled guilty to a felony count of “willfully and knowingly” making false statements to the FBI. He was spared trial on related charges when Mr. Trump pardoned him in 2020, but was a high-profile scalp for Democrats, nevertheless.

On Thursday, a Fox News contributor, Byron York, posted on X that he’d “argued … during the Michael Flynn matter that the Logan Act is a dead letter. But politically,” Logan’s forte, “it’s useful to know that Senator Van Hollen traveled to a foreign country to bash the President of the United States.”

Republicans, smarting from Logan’s use against Flynn, want payback. Expect growing calls for them to employ Washington’s favorite MacGuffin, flipping the script on Democrats by launching investigations of all those lobbying for Mr. Abrego Garcia.