FORGET Walter Mondale. If you want proof that the Democratic Party is in trouble -- deep trouble -- just look at what happened in the U.S. Senate last week.

While the Republicans were electing an independent and innovative Robert J. Dole of Kansas to be their majority leader, while they were choosing moderates as committee chairmen, the Democrats stuck with leaders who can be charitably described as uninspiring. And unlike Fritz Mondale, these pols aren't about to go away.

Just compare Dole to his Democratic counterpart, Minority Leader Robert C. Byrd (W. Va.). With Republicans in control of the White House and Senate, Byrd could have been a spirited voice of opposition. But how often have you heard him speak out over the past four years? Not very.

In 1981, he threw in the towel and voted for the Reagan budget and tax cuts. These days, he routinely votes against the administration -- but doesn't offer any alternatives. On tax simplification, for example, you don't see Robert Byrd embracing the Bradley-Gephardt bill -- let alone advocating a plan of his own. By allowing the Republicans to draw the legislative battle lines, Byrd surrenders before the war begins. He fights on their turf.

Surely, no one expects Byrd to be a latter-day Churchill -- a thunderous leader-in-exile. But is it too much to ask that he and the Democrats build their own agenda?

Ironically, it is Dole -- once dismissed as the GOP's Don Rickles -- who emerged as a leader unafraid to buck party and president. In 1982, when the deficit started to expand out of control, Dole proposed a tax hike. The White House fought him; columnist William F. Buckley and the Republican Right lambasted him. But he stood his ground -- and pushed through a loophole-sealing bill that Ralph Nader's People & Taxes hailed as the best tax reform in memory. That's leadership.

It might be okay if crusty Democrats held one or two top slots in the Senate. But like cholesterol in the blood stream, they're everywhere. At Foreign Relations the party is led by Claiborne Pell (R.I.), a senator who has solid liberal credentials but who is so lethargic staffers regularly call him "Stillborn." He's no match for incoming chairman Richard G. Lugar (Ind.).

Democrats at Appropriations could also stand a little Geritol. The current chairman is Mark Hatfield (Ore.), liberal Republican; the ranking Democrat is John C. Stennis (Miss.), 83, Homo Dixiecratus.

On defense spending, the Hatfield difference is conscience. In 1966, his was the lone vote against a National Governors Conference resolution supporting the war in Vietnam. And in the Senate, Hatfield not only co-authored the nuclear freeze bill, he led Republican opposition to the MX and nerve gas. For 12 years, Stennis had exceptional power as chairman of Armed Services and Defense Appropriations. He could have brokered for defense reform; he never opposed a major weapons system.

The Democratic leadership on the Finance Committee is just as enlightened. The chief Democrat is still Russell B. Long (La.), the self- proclaimed "darling of the oil industry." Anyone looking for tax simplification best look elsewhere. During his 15-year reign as chairman, Long's most progressive measure was the Reform Act of 1969 -- a bill laden with 200 special provisions including exemptions for Mobil, Uniroyal and McDonnell-Douglas.

"I suppose these tax breaks really are tax expenditures," he told the committee in 1974. "I've never been confused about it. I've always known that what we were doing was giving away government money." Is this a voice of the Democratic future?

The new Republican chairman, Bob Packwood (Ore.) may not be a lot better -- he has supported tax expenditures and isn't a promoter of tax simplification either. But, at 52, he's 14 years Long's junior, and he seems more independent of the special interests.

Even at the Environment and Public Works Committee, the Republicans outshine the Democrats. The current chairman, Robert T. Stafford (Vt.) has been a loyal ally of conservation groups. The League of Conservation Voters, for one, rates him twice as high as his would-be Democratic successor, Lloyd Bentsen of Texas. Unlike Stafford, Bentsen has opposed supplementary funds for the Superfund cleanup of toxic waste, home weatherization and the Youth Conservation Corps.

True, at most committees the ranking Democrats aren't any worse than their GOP chairmen. They're just an echo. J. Bennett Johnston (La.) uses his seat on Energy to support his state's oil and gas interests. He often votes with Republican Chairman James A. McClure (Ida.) Lawton Chiles (Fla.), the Democratic leader at Budget, is as fically conservative as Chairman Pete V. Domenici (N.M.. And at Commerce, Democrat Ernest Hollings and soon-to-be Chairman Robert Danforth (Mo.) are both champions of auto safety legislation. Calling himself a "cheap hawk," Sam Nunn (Ga.) uses his ranking Democratic seat at Armed Services to vote with Barry Goldwater (R-Ariz.) for most major weapons systems.

To be sure, the Republicans are not without their conservatives: Jesse Helms (N.C.) and Strom Thurmond (S.C.), for example. Nor are the Democratic leaders without shining lights. Joe Biden (Del.) and Ted Kennedy (Mass.), ranking Democrats at Judiciary and Labor, stand out. But the sad truth is that most Democratic leaders are about as vivacious as the Omaha Chamber of Commerce. And about as liberal, too.

Fortunately, once the old-timers move on, the Democratic future will belong to younger senators such as Bill Bradley (N.J.), Dale Bumpers (Ark.) and Patrick Leahy (Vt.). Compassionate and effective, they tower over their party leaders. Democrats can also be proud of their latest recruits. Sens. Tom Harkin (Iowa), Albert Gore Jr. (Tenn.), Paul Simon (Ill.), John F. Keery (Mass.) and John D. Rockefeller IV (W. Va.) will surely add life to the party. But it will be years before they take the place of the old guard.