FOR THE PAST couple of months, it has seemed Democrats would hold the Senate in next year's elections. Now the balance has shifted a bit. Retirement and candidacy decisions this fall helped the Democrats. Paul Trible's retirement opened up the Virginia seat for former governor Charles Robb, while Chic Hecht trails far behind Gov. Richard Bryan in Nevada. Former governor Bob Kerrey, once he decided to run, is the favorite to take the Nebraska seat held by appointee David Karnes. These are three examples of governors, current or former, whose popularity is strong and deep running against Republicans who are not known in anything like that depth. If you assume that the Democrats win these three seats, and remember that the Republicans must make a net gain of five to hold the Senate, then it looks like the contest for the Senate was pretty well decided by Thanksgiving.

Or looked. For the retirement and candidacy decisions of the past couple of weeks tend to favor the Republicans, giving them a chance to win several Democratic-held seats that used to look safe. William Proxmire's retirement opened up the race in Wisconsin, where the Republicans showed strength by winning the governorship in 1986. John Stennis' retirement opened the way for House Republican Whip Trent Lott in Mississippi; he will have serious Democratic opposition but is probably the favorite now. Lawton Chiles' surprise announcement this week that he'll retire -- he has raised $1.3 million and spent Thanksgiving weekend walking 60 miles to meet voters -- leaves the major announced candidate as Republican Rep. Connie Mack III: this will be a free-for-all, but the Republicans' chances have improved.

In North Dakota the nonretirement of 79-year-old incumbent Quentin Burdick and the refusal of popular Rep. Byron Dorgan to challenge him for the Democratic nomination give the Republicans a chance to use the age issue Mr. Dorgan was starting to raise; in Ohio the withdrawal from the race of Rep. Bob McEwen gives Cleveland Mayor George Voinovich the Republican nomination unopposed and improves his chances to beat Howard Metzenbaum. Finally, in New York U.S. Attorney Rudolph Giuliani is considering a race against Daniel Patrick Moynihan, who did not have serious opposition in 1982.

All of this leaves the Republicans facing an uphill battle. But their odds of holding 45 seats in the Senate or making some gains have been improved, and any gains will put them in a far better position to regain control in 1990. They'll need all the breaks to win this time, but sometimes a party -- the Republicans in 1980, the Democrats in 1986 -- gets nearly all the breaks and wins control. For more than a quarter of the century, until 1980, the Senate seemed eternally Democratic. Now it seems perpetually up for grabs.