An update has been made to this story.

Campbell Ewald is in talks to pick up the Cadillac account, Crain's Detroit Business is reporting.

A deal has not yet been finalized and financial terms have not been disclosed, but the account for the General Motors brand is believed to be valued at about $244 million annually. No timeline has been disclosed.

The talks have been characterized as serious, and the account shift would represent a relationship renewal between two companies linked since the auto industry's infancy.

GM sent Ad Age a statement on March 12 confirming that the account was in review but did not identify Campbell Ewald as a front-runner or contender. "Cadillac is conducting an advertising-agency review over the coming weeks," GM said in a statement to Ad Age . "Publicis Groupe 's Fallon Worldwide, which is the incumbent on the business, remains our current advertising agency of record; no change has taken place."

The automaker added that "Fallon will be invited to participate in this review process along with other agencies," but said it is "not disclosing the other participating agencies nor details of the review process."

Campbell Ewald handled Chevrolet's marketing for General Motors for 91 years until former CMO Joel Ewanick established Commonwealth, an unprecedented relationship between holding company rivals Omnicom Group and Interpublic Group of Cos., to take on Chevy globally. Ad Age reported this week that McCann is taking the lead in Commonwealth, essentially driving out the other partner, Goodby Silverstein & Partners.

The Cadillac scenario, as laid out by insiders and explained to Crain's Detroit Business, has Campbell Ewald eventually taking over Cadillac from Fallon Worldwide, Minneapolis. Cadillac landed at Fallon in June 2010 after Mr. Ewanick shifted the account from Bartle Bogle Hegarty, which had the business for only six months.

Campbell Ewald Chairman-CEO Bill Ludwig referred calls for comment to Cadillac via his agency's outside media-relations firm, New York City-based DiGennaro Communications. David Caldwell, manager of communications for Cadillac, declined to comment. An email to Fallon 's Detroit office this afternoon has not yet been returned, and a spokeswoman at the agency's corporate headquarters referred questions about the Cadillac account to a colleague who has yet to return a call.

People familiar with the situation said Campbell Ewald was hired by Cadillac this year to do an assessment of the brand's entire marketing effort -- basically one ad agency auditing the work of a rival on an account it covets.

The Cadillac deal is believed to have been in the works for several months, and the likelihood of a contract may have been the impetus for Campbell Ewald to ink a much-publicized 11-year lease in Detroit, where it will be moving from its Warren, Mich., headquarters. Campbell Ewald has about 700 staffers now, down from more than 1,200 five years ago. About 600 are expected to make the move downtown when the Ford field office is ready by the end of the year, and the remainder will move once the Warren building is sold, Mr. Ludwig said last week.

Chevrolet meant $30 million in revenue for Campbell Ewald based on $600 million in media billings in 2009. The agency had estimated U.S. revenue of $121 million in 2011 and is the nation's 66th largest ad firm by that metric, according to Ad Age 's DataCenter . In 2007, it brought in an estimated $239 million in revenue.

In October, Mr. Ludwig told Ad Age that a third of the Chevy business had been replaced. The account at one time was 80% of the agency's billings, but was down to 25% when the work was lost in 2010.

Overall, the automaker spent $1.7 billion marketing all of its brands in 2012, down from $1.8 billion the year prior. Cadillac's sales declined 2% last year to 149,782 units, the first fall-off after two years of year-over-year sales increases, according to information provided by Automotive News. Sales peaked in 1978 with 350,813 vehicles sold.

Cadillac's share of the U.S. sales market fell to 1% last year from 1.2% in 2011 and 1.3% in 2010.

This is the first major agency move by Alan Batey, who became GM's interim chief marketing officer in July after the automaker fired Mr. Ewanick, who was ousted over questions about how marketing money was being spent.

Former GM CMO Mark LaNeve dumped Cadillac's ad agency, the Troy office of Chicago-based Leo Burnett, in 2007 without a review and parked the work at Boston-based Modernista to do the creative work. Leo Burnett had held the Cadillac account since 1935.

The New York office of Bartle Bogle Hegarty got what was a $270 million Cadillac advertising account in January 2010 after Modernista chose not to defend the account in a review handled for GM by Ark Advisers.

Leo Burnett now handles advertising for Buick, GMC and the Chevy Silverado at its Troy office and its Chicago headquarters.

Fallon Worldwide, a subsidiary of Publicis Groupe , is believed to have about 50 employees on the Cadillac account in offices at the Renaissance Center, which is GM's headquarters. Fallon , whose other major clients include Purina, Travelers and H&R Block, jettisoned Chrysler's marketing work after less than a year so it could handle Cadillac.

General Motors in 2012 announced an effort to consolidate its roster of 70 creative agencies in a bid to save $2 billion over the next five years. That projected savings from streamlining both advertising and media-planning efforts represents, on average, $400 million a year. That's about 10% of GM's global advertising expenses of $4.48 billion last year.

In January 2012, GM gave its $3 billion media-buying and planning to Aegis Group's Carat. The Detroit office of digital marketing firm MRM Worldwide, which has the same corporate parent as Campbell Ewald, already does Cadillac work and is expected to continue to do so with Campbell Ewald.