WITH LAST Tuesday's primaries, the lineup in this fall's 34 Senate races is all but complete. Both parties' nominees have been selected or are apparent in all but four states; only Democrats in Georgia, Maryland and Wisconsin, and Republicans in Colorado have yet to choose. Still, the critical question -- will the Republicans hold the Senate or will the Democrats take it? -- seems far from decided.

By our reckoning, in the past 17 months the balance of pundit opinion has shifted several times. First it was thought the Democrats would win, because they had only 12 seats up to the Republicans' 22. Then it was thought that Republicans would prevail, after veteran Democrats Russell Long and Thomas Eagleton quit. Then it looked like the Democrats again, with Paul Laxalt's withdrawal. Then the Republicans again, as strong young Democratic congressmen declined to run. Now conventional wisdom says the Democrats are ahead again, because Sen. James Abdnor beat Gov. Bill Janklow in the South Dakota Republican primary.

Not national issues but local and personal factors seem to be making most of the difference. The Senate is primed to vote 100 to 0 on tax reform and has not been the scene of great disputes on the budget. So the oddsmakers check to see how Sen. Paula Hawkins' back is doing and whether Sen. Robert Kasten's drunk-driving incident is having much impact. Is Sen. Alan Cranston, born three years after Ronald Reagan, too old? Will voters learn to pronounce the last name of his Republican challenger Ed Zschau? (It's the first syllable of shower.) Since Sen. Charles Grassley started attacking Reagan administration farm and defense procurement policy, senators of both parties have been campaigning pretty much on their own, rather than on their parties', platforms.

That's true even though the Republicans, in their six years in control of the Senate, have made a generally constructive and coherent record on important issues, a record distinguishable from that of the Reagan administration, from the House Democrats and from the Senate Democrats too. All of which suggests a good question for Democratic Senate candidates: what would you have your party do if it wins control of the Senate next fall, and how would you do it?