Richard P. Sullivan, a Baltimore business executive, announced yesterday that he would run for the Republican nomination for the U.S. Senate, arguing that his status as a businessman who is outside the political mainstream will work to his advantage.

Sullivan is the second Republican to jump into the race for the seat being vacated by Republican Sen. Charles McC. Mathias Jr., but Del. Thomas Mooney of Prince George's County, who announced several weeks ago, has hinted that he may pull out and run instead for governor. Tomorrow, former White House aide Linda Chavez is scheduled to announce her candidacy for the Senate, with a five-stop sweep through the state.

"I'm not a politician. I'm a businessman. When I made decisions, right or wrong, they were my responsibility, and I had to live with the consequences," Sullivan said in his announcement at his Baltimore headquarters. "America faces tough decisions, and we need some senators who understand the consequences."

Sullivan, 52, was chairman and chief executive officer of Easco Corp. for 12 years, until he was forced out last summer during a hostile takeover bid. He has never run for elective office and is not well known in Republican political circles.

Although neither Chavez nor Sullivan has been involved in Maryland GOP politics, party officials appeared pleased to have two active candidates after months of searching. Mathias was first elected to the seat in 1968, but the overwhelming Democratic registration in the state has discouraged some potential GOP candidates.

Chavez has spent weeks courting Maryland Republicans and seems to be better known among party activists. But Sullivan said he believes that voters will prefer him because "coming out of the private sector, where there is a focus on problem-solving, I have quite a different perspective" than Chavez.

Sullivan acknowledged that Maryland voters "don't know me yet," but both Republican and Democratic insiders have said he could overcome that if he is willing to spend a substantial amount of money on a media blitz.

Sullivan said yesterday that he plans to spend $100,000 of his own money, and a total of $3 million, in the primary and general elections. That is comparable to the amount targeted by better-known Democratic candidates. Sullivan said he has raised $60,000 in three weeks.