Rep. Roy P. Dyson of Maryland, seeking to fend off a field of 10 challengers and lingering questions regarding campaign contributions and the operations of his office, formally opened his campaign for a sixth term yesterday, saying he offers "a strong record of effective service."

Dyson, a Democrat who represents the sprawling, predominantly rural 1st District, began his campaign with a two-day speaking tour that will take him from Worcester County on the Eastern Shore to the Pennsylvania border. Dyson is being challenged by two other Democrats in a Sept. 11 primary; eight Republicans are vying to take him on in November's general election.

"I welcome this opposition because it gives me an opportunity to tell the Roy Dyson story -- a story of effective service to the 1st Congressional District," Dyson said. "Friends, I've always put Maryland first. I am asking to be reelected to the Congress based on a strong record of effective service to every county in the 1st District."

Political analysts say Dyson is particularly vulnerable because of several incidents involving Tom Pappas, who was Dyson's administrative assistant until he committed suicide two years ago. The Federal Election Commission fined Dyson $3,000 after it found that Pappas had pocketed about $7,000 in campaign funds. Pappas killed himself after reports of unorthodox activities in the operation of Dyson's office.

Dyson, 41, also refunded campaign contributions made by several defense industry contractors after they were implicated in a bid-rigging scandal.

Dyson narrowly won reelection in 1988 over Republican Wayne T. Gilchrest, who had no political experience. The National Republican Congressional Committee has targeted Dyson as one of the incumbents it has the best chance of knocking off.

The Democrats challenging Dyson are Del. Barbara Osbourn Kreamer and lawyer Michael Hickey Jr., both of Harford County. The GOP contenders are Gilchrest, Richard Colburn, Mark Frazer, Luis Luna, Perry Weed, Ray Briscuso, Barry Sullivan and Charles Grace.