THOUSANDS of Republican delegates are planning to take in a Broadway show Sunday afternoon – but will the actors show up for work?

There is a campaign, fanned by anti-Republican groups on the Internet and being discussed informally among the actors themselves, encouraging performers to boycott the Republicans by calling in sick on Sunday.

That’s the day the Republican National Committee has bought up blocks of tickets to such shows as “Fiddler on the Roof,” “42nd Street,” “Wonderful Town,” “The Lion King” and “Aida.”

It is impossible to know how effective the campaign is at this point, but producers are concerned enough about it that some are planning to meet with their casts tomorrow to remind them of their contractual obligations.

“It’s out there,” one theater executive said yesterday of the possible boycott.

“We’re picking it up. The actors are talking about it.”

In e-mails and in letters posted backstage at theaters this week, Actors Equity has also told Broadway performers that they are expected to work Sunday.

“Actors Equity . . . has advised its members working on Broadway to report to work as usual during the upcoming Republican National Convention,” the letter reads.

Alan Eisenberg, the head of the union, said in a statement: “Our members are consummate professionals and will, pursuant to our collective bargaining agreement, honor their obligations to our employers.”

A Web site called Shut It Down.com claims it has launched a letter-writing campaign to actors “asking them to take the night off” from shows the Republicans will be attending.

The Post could not find any Broadway actors who had received letters from the organizers of Shut It Down.

But some actors, speaking on condition of anonymity, said talk of what one called “the blue flu” was making the rounds.

“This is a very liberal community,” the actor said. “A lot of people are not happy about having to perform for Republicans.”

A top Broadway producer expressed outrage yesterday that the actors were even considering such a boycott.

“I find the whole thing offensive,” the producer said. “All I can say is that the people in our show had better be prepared to do their jobs on Sunday.”

The actors aren’t alone in their unhappiness about having to entertain Republicans.

Backstage crews may be registering their displeasure as well.

At “42nd Street,” the word is that a lot of behind-the-scenes staffers are going to wear anti-Bush T-shirts to the show on Sunday.

How effective that particular way of protesting will be is somewhat doubtful, since there won’t be many Republicans hanging around backstage during the show.

“Maybe they’ll see us outside the stage door during our smoking breaks,” jokes one “42nd Street” wag.

Broadway producers and theater owners are also concerned about possible protests in Times Square.

Another Web site, indymedia.org, is calling for a “mouse bloc” – groups of protester “mice” to scare Republican “elephants” at what the Web site describes as “Disney-fied” Broadway shows.

The Web site is urging protesters to show up at Broadway theaters before the shows start and “remind the forgetful old elephants that they are not welcome in this town.”

The Web site contains the names and addresses of all the theaters housing shows the Republicans are attending.

Theater owners met with police officials earlier this week to discuss security measures and the possibility of protests.

“Truthfully, I think we’re more concerned about protesters than we are about terrorists right now,” one theater executive told The Post yesterday.