WHEN REPUBLICAN Charles McC. Mathias Jr. ran for reelection to the Senate against Democrat Barbara Mikulski six years ago, Maryland's voters were treated to an interesting campaign -- an intelligent discussion of state, national and foreign affairs by two smart, well-informed people. But any similarity between the strong and thoughtful Democratic challenge waged then by Miss Mikulski and that being conducted today by State Sen. Edward T. Conroy is purely coincidental. What have been passed off as "debates" between the two men were cruel mismatches, both underscoring the shortcomings of challenger Conroy as a replacement for Sen. Mathias.

As a state legislator, Mr. Conroy enjoyed the friendship and appreciation of many of his Annapolis colleagues, as a man who was hard-working, fair in his role as a committee chairman and an able representative of various blocs of interests -- including veterans, anti-abortionists, senior-citizen groups and organized labor. But in switching political gears to run for the U.S. Senate, Mr. Conroy is the first to note that he has emerged "to the right of Mathias, and that's exactly where I want to be." What is still not clear, though, is exactly where this would take him or his constituents were he to win a six-year term in Washington.

Mr. Conroy has tried to make defense a main issue in the campaign, alleging that the voting record of Sen. Mathias somehow suggests a weak stand. Yet a close examination of these votes shows nothing of the kind. Instead, Mr. Mathias has commented, "I say to you with some sorrow that in my 12 years I have voted for $1.1 trillion for defense of the United States of America. I wish that sum could have been spent for other purposes, but I thought it necessary."

Responsbile, independent and energetic thinking on issues has served Sen. Mathias and Marylanders extremely well on Capitol Hill. And it has won him a position of considerable influence in Congress, with respect from both sides of the sides of the aisle for his ability to weigh matters on their merits, his courage and his ease and humor. Not only has his sponsorship and management of legislation spanned a broad range of subjects, but Sen. Mathias has also kept intact a sense of local and state interests and a high record for old-fashioned constituent service in its finest sense.

Little wonder, then, that the senator has earned bipartisan support around the state. "Mac" Mathias has managed successfully to resist the shallow political appeals of extremists for quick-fix decisions, preferring a reasoned approach to problems ranging from world population to nuclear energy and the environment, to foreign trade and the economic health and well-being of Marylanders.

As we said, this is just no contest, Sen. Mathias should be reelected.