The likely favorites in tonight's two divisions of the $103,190 William E. Miller Memorial Pace at Rosecroft Raceway represent the old and new worlds of harness racing.

Elarfus, in the first division, was bought at public auction for $295,000 in January by the same stable that owns Cam Fella, 1982's harness horse of the year. Elarfus is the seventh-ranked 3-year-old on this year's experimental list of the U.S. Trotting Association.

A contrast is provided by GE's Romanero in the second division. His owners, Eugene Ewing and his son, Gary, used to be soybean and corn farmers in Easton, Md., and they did a little harness racing like many of the other farmers on the Eastern Shore.

In 1973, they bought an unraced 2-year-old filly named Romeo's Belle for $2,700 from the renowned Stanley Dancer. As a racehorse, Romeo's Belle won more than $130,000. With another successful race mare named Theresa Lobell, who broke the Laurel Raceway track record in Gary Ewing's 13th career drive in 1978, the Ewings decided to rent the crop land and concentrate on breeding and racing.

They now have 25 to 30 horses, mainly broodmares and young animals; only five are racing. Theresa Lobell's first foal, a Nero colt named GE's Rebel, is expected to race late in the year.

For a $30,000 stud fee, Romeo's Belle's first mating was to Nero, a champion pacer in the mid-1970s, and the offspring was GE's Romanero. "She was a nice race horse and she throws 'em like she was--she could get away from the gate and leave fast," said Gary Ewing, now 25.

Tonight, Ewing will be competing in his first $100,000 race, and GE's Romanero, like his dam, will be trying to get the lead. The colt has won six of his last seven races, with a second-place finish against older invitational horses here his only loss. In each victory, GE's Romanero had the lead at each call of the race.

GE's Romanero will start from Post No. 5 and will have to contend with Keystone Exceller, one of three horses trained by Jim Miller, who won the 1979 Miller Memorial with Tijuana Taxi. Keystone Exceller's $174,154 in earnings represent the highest total in the 14-horse field.

Each division is worth $51,195. In the first division, Elarfus, with a third-place finish in three starts this year, may need one more race before achieving the form that enabled him to finish second to Trim the Tree in a 1:53 3/5 mile last year at Lexington.

Mountain Fella, another of the Jim Miller-trained entry, probably will be second choice, but could emerge as the betting favorite. Most of his success has come on a half-mile track, including a third-place finish in the $510,400 Peter Haughton Memorial last year and a victory at Freehold this year in 1:58 3/5. Another contender is Cuidad Trujillo, a strong closer.