For more than a century, The Force -- as in the force of law -- has been with open space advocates as they battled to keep Chicago’s lakefront free of buildings.

In 1909, Aaron Montgomery Ward won a historic court fight that kept the Field Museum out of Grant Park. Soon after he was elected mayor in 2011, Rahm Emanuel cited the same legal precedents as Ward and helped kill former Mayor Richard M. Daley’s plan to insert the new home of the Children’s Museum of Chicago in Grant Park.

Now, as Emanuel revels in George Lucas’ selection of Chicago over Los Angeles and San Francisco as the site of the moviemaker’s narrative arts museum, the mayor and the “Star Wars” creator face the prospect of a battle royal with lakefront advocates who argue that the plan violates Chicago’s landmark Lakefront Protection Ordinance.

That law was passed in 1973 after the construction of the 70-story Lake Point Tower and the McCormick Place convention center east of Lake Shore Drive. It charges members of  the Chicago Plan Commission, some of whom are appointed by the mayor, with determining whether development proposals adhere to the city’s 14 policies for the shoreline. One of those policies stipulates that “in no instance will further private development be permitted east of Lake Shore Drive.”

 That provision is the likely basis on which lakefront advocates could mount a legal attack on Lucas’ plans for a museum, which would house his collection of Norman Rockwell paintings and memorabilia from “Star Wars.”

“We will do what it takes and that very well may be a lawsuit,” Friends of the Parks’ President Cassandra Francis said Tuesday night.

Through a spokeswoman, Emanuel on Wednesday said that the planned the museum would survive the anticipated legal challenge because it would be publicly, not privately, owned, and therefore in accord with the Lakefront Protection Ordinance.

While the museum would be designed and built with private funds, it would be “turned over to the [Chicago] Park District upon completion,” mayoral spokeswoman Kelley Quinn wrote in an email.

In addition, by transforming 12 acres from parking lots to parkland, the Lucas museum would meet the mandate of the Lakefront Protection Ordinance to expand “the quantity and quality“ of parkland along the shoreline, Quinn said.

“The Lakefront Protection Ordinance requires development to meet a number of thresholds all of which would be met with the proposed Lucas Museum,” Quinn wrote.

Lucas is expected to spend hundreds of millions of dollars on the museum, and it is not known if he has agreed to Chicago Park District ownership of the building that would bear his name. Devon Spurgeon, a spokeswoman for the moviemaker, declined comment.

Gary Johnson, president of the Chicago History Museum and of Museums in the Park, a group of 11 museums located on Chicago Park District land, said the typical arrangement is for private, non-profit institutions to own their buildings and pay the park district a nominal fee for use of the land.

“My understanding is that (the Lucas Museum will have) the same arrangements we have,” Johnson said. “The nonprofit that is the Chicago Historical Society owns our building.”

Visionary planners like Daniel Burnham often get the credit for shaping the grand public spaces of Chicago’s lakefront, but laws—and legal sleights of hand—have had an equally powerful impact.

Ward won his court battles by citing a map of Grant Park drawn up by three commissioners that the state of Illinois named to supervise the construction of the Illinois and Michigan Canal. As they laid out the canal lands in 1836, they left a strip of land in what is now Grant Park as open space, writing on the lakefront edge of their map: “Public Ground—A Common to Remain Forever Open, Clear and Free of any Buildings or other Obstruction Whatever.”

But there have been exceptions to this famous prohibition.

In the 1890s, The Art Institute of Chicago was allowed to build its neo-classical main building in Grant Park because adjoining property owners signed consent forms allowing construction. The practice continued with the museum’s Renzo Piano-designed Modern Wing addition in 2009.

Before Millennium Park’s 2004 opening, city officials labeled the park’s Frank Gehry-designed Pritzker Pavilion a “sculpture” to evade the prohibition against new buildings that obstruct views in Grant Park.

The 17-acre, North Burnham Park site that Chicago has offered Lucas for his museum is occupied by two parking lots located between Soldier Field and the lakefront building of McCormick Place. The first, the Waldron parking deck immediately south of the stadium, has 1,500 spaces. The second, farther south, is a surface parking lot with 1,300 spaces. On Tuesday night, a few seagulls camped out on its cracked black asphalt.

Citing such ungainly scenes, Emanuel portrays the Lucas Museum as a way to complete the Museum Campus, a lakefront greensward that was created in the mid-1990s when Lake Shore Drive’s northbound lanes were moved off the lakefront and west of Soldier Field.