Emersons, Ltd., the steak and salad restaurant chain that got into legal and financial difficulties more than a year ago, has launched a flamboyant advertising campaign in an attempt to boost lackluster sales.

The Rockville firm last year filed for voluntary reorganization under Chapter XI of the Bankruptcy Act. Rearrangements with its principal creditor. Fidelity Bank of Philadelphia, still are being negotiated. In the intervening months, says Emersons' President Bill D. Jackson, the company pared its outlets to 21 - half the numbers it once had.

The firm went through a most difficult year in 1977. Despite concentration on improving operations, sales are essentially flat, although they have improved a bit recently. At the same time, Emersons borrowed and since has repaid, $200,000 on a $4 million line of credit Fidelity extended to it.

To counter the numbing effect on sales of the long, cold winter, Emersons came up with what it believes is a red hot ad campaign.

The ad reads, in part: "Strip away from Sans Souci, Le Bagatelle and Rive Gauche the gloating Maitre d's, smug waiters, more-tasteful-than-thoudecor. steak dinner for $15 plus . . . and you have the same dinner at Emersons for $7.50." It pictures a without trousers wearing a T-shirt marked "Property of Sans Souci" and holding a menu in front of him to cover his nakedness.

In smaller print Emersons explains it can charge cheaper prices because it buys its beef directly from the Midwest, thereby cutting out the expenses of the middlemen.

The ad is the creation of Stanley Cotton, a marketing man who once tried a convince the Democratic National Committee to raise funds through a national sweepstakes and who also filled jars with $10,000 in shredded bills and sold them through the mail for $6.

Cotton considers Emersons' tongue-in-check ad a "compliment" to the French restaurant named. "It means we picked those three as the top restaurants in town," he said. He termed the ad a "Hertz and Avis" situation.

At first, The Washington Post declined to run the ad on the grounds it "crossed the line of acceptability, making derogatory and derisive comments about competitors," according to Christopher Little, the company's lawyer.

The reaction of the owners of the other restaurants mentioned ranged from indignation to amusement. Paul Zucconi, co-owner of Le Bagatelle, said his lawyer had advised him he had no legal recourse. "I'm not angry. It won't make any difference. We must let the customers make their own comparison.

The Post subsequently agreed to run revised version of the ad. In place of the three French restaurant names and the $15 pricetag, the word "Censored" appears. An asterisk notes that an"un-censored version of the ad can be seen at Emersons restaurants.