WHEN a politician of the stature of Sen. Charles McC. Mathias Jr. steps aside from another ordeal of reelection, his decision is gone over with a fine comb of analysis. Are the GOP moderates, among whom Mr. Mathias was in the front rank, an endangered spcecies? Was it simply the prospect of a strong challenge by Maryland's Democratic governor, Harry Hughes, or Democratic Reps. Michael Barnes and Barbara Mikulski? In Mathias's case, given his strong, independent judgment on civil rights, national resources, campaign spending, foreign policy, and constitutional issues like the balance of powers, one is led to accept his own explanation that it was time for a new phase of his career, rather than to conclude that his moderate stance was out of phase with the times.
Indeed, it is at the moderate center in the Senate where one finds today's action. Senate majority leader Robert Dole, Mathias's classmate in the Senate class of 1968, and others like Sen. Bob Packwood, Finance Committee chairman, have taken an independent lead in a host of areas -- defense spending, social legislation, South African sanctions -- where the White House was bogged down.
When one looks at the half dozen retiring from the Senate at the end of 1986, no set pattern emerges. Sen. Russell Long of Louisiana first joined the Senate in 1948. Sen. Barry Goldwater of Arizona has served a full career. Sen. Paul Laxalt of Nevada has perhaps a presidential run in sight.
There are at least four key factors in the 1986 election which on balance lean toward the Democrats: Of the 34 contested seats, 22 are held by Republicans, and of these, 16 of the incumbents were first elected in 1980; no fresh broom will be sweeping in 1986 like the one that carried Jimmy Carter and Senate Democrats out of office in 1980; Republican party-line discipline in the Senate is fading, as incumbents look to agricultural, trade, or other special interests of their states, frequently at odds wi th the Reagan administration priorities; and strong challenges are emerging from among the governors and congressmen of both parties.
These 1986 midterm elections will offer the first glimpse of the post-Reagan political era. The issues seem to be more local than national -- of protecting one endangered state industry like timber in Oregon -- without a broad nationwide or international theme like the case for a defense buildup in 1980.
At the same time, the personal costs of running for high office continue their climb: Campaigns are more intense and take longer periods of time; messages to constituents get harder to craft; and the burdens of criticism and negative advertising become more onerous.
Washington does not stay the same for long. Even acknowledging this, however, will not make public servants like ``Mac'' Mathias any the less missed.