WHAT DOES a conscientious Maryland voter look for when sizing up this year's candidates for the U.S. Senate? Contests for the Senate tend to be the most difficult to assess: would-be governors can talk about state issues, and candidates for the U.S. House can cater their menus to the more readily discernible concerns of their districts. But what should the candidates for the Senate be emphasizing -- military aid for the contras? Saving the rockfish in the bay?

"Liberal" or "conservative" handicapping doesn't prove much, at least not before the party primaries. Sen. Charles Mathias, who is vacating the seat, has always had a special philosophical blend of his own that got him over the toughest hurdles -- his own Republican Party's primaries -- as well as the general elections in a state with an overwhelming Democratic majority. But how that will affect the intraparty race between Republicans Linda Chavez and Richard Sullivan (and still other Republicans who are now thinking about jumping in) isn't at all clear.

Mr. Sullivan did commission a poll by Decision Making Information, a firm used by President Reagan, of more than 400 registered voters -- and, yes, it shows him leading his primary opponent 24 percent to 17 percent among Republican voters -- with a critical 59 percent undecided. That's hardly definitive; on the contrary, that race may be wide open.

True, say many Democrats, but Republicans are so outnumbered that any one of the four Democratic candidates -- Harry Hughes, Barbara Mikulski, Michael Barnes or Donald P. Hutchinson -- could take the general election. But here again there are some intriguing preliminary findings in the Sullivan polling:

Among all voters surveyed, 53 percent said they believed the Republican Party is better at "keeping the economy growing"; 57 percent said the GOP is better at "fighting inflation"; 64 percent said the Republicans are better at "keeping interest rates low." "Furthering civil rights" was another matter: a total of 69 percent said the Democrats do that better.

These are big issues, but will they make the difference? Don't be too sure. Other things can matter much more. The largest majority in the Sullivan poll -- 89 percent -- agreed that "someone should live in Maryland for more than two years before" he or she runs for office in the state. Now there's an issue.