With Ms. Weidel’s appearance at Oxford still a week away, some influential members of the Oxford community have come to the Union’s defense.

“I take the classic liberal position that it’s better to have the open debate, have them be challenged, and then often that ends up demolishing and exposing their position much more effectively than anything else,” said Timothy Garton Ash, a European historian at St. Antony’s and one of the world’s leading public intellectuals.

The hourlong event will include a brief speech by Ms. Weidel, 20 minutes of questioning by the Oxford Union’s president, Stephen Horvath, and 20 minutes of questioning by the audience. The Union has already contracted additional security for the event, Mr. Horvath said, and will consider canceling the event only if Ms. Weidel herself decides to withdraw or if the police instruct the Union to do so for security reasons.

Mr. Garton Ash added that just as he supports hosting Ms. Weidel so long as she is “challenged rigorously,” he also supports community members who choose to protest the event.

Already, more than 550 people have expressed interest online in attending a demonstration against Ms. Weidel’s appearance. Ian McKendrick, an officer of Stand Up Against Racism, one of the groups organizing Wednesday’s demonstration, said protesters would congregate outside the Oxford Union. Their goal, Mr. McKendrick said, is both to “demoralize racists” and “embarrass the Oxford Union” for extending such an irresponsible invitation.

“Inviting her to speak was a gross mistake,” added Kate Douglas, an officer of Unite Against Fascism, another of the groups organizing the demonstration. “It’s giving her a platform. It’s making her views more mainstream and acceptable, and we know what will follow: race hate, racist attacks and the legitimization of racist views.”

The United States has seen a similar debate about featuring incendiary figures on prominent platforms, including the Alt-Right leader Richard Spencer and Mr. Bannon. In September, when it was announced that Mr. Bannon would speak at The New Yorker’s annual ideas festival, other participants pulled out of the event. Under immense pressure, David Remnick, the editor of The New Yorker, withdrew Mr. Bannon’s invitation.