The death of Georgia Republican Paul Coverdell in July shifted the balance in the Democrats' favor because former Democratic Gov. Zell Miller was named to replace him. The Senate GOP majority was thus trimmed from five seats to four. With eight vulnerable Republicans up for election in November and one seat left open by a GOP vacancy, the Democrats began smelling victory.
Then came Joseph I. Lieberman.
Vice President Al Gore's selection of the popular Connecticut senator as his running mate is now seen as a setback for the Democrats' Senate campaign. If Gore prevails in the presidential election this November, Connecticut's Republican governor, John G. Rowland, will select Lieberman's replacement - almost certainly a Republican.
And the way the election is shaping up, that one Republican seat could be enough to deprive the Democrats of a majority.
"I can paint a scenario that gives the Democrats the majority, but it would require a change in the political landscape," said Jennifer E. Duffy, who tracks Senate races for the Cook Political Report. "We think the Democrats are going to gain two or three seats. Gaining four could be done, if they have an exceptional night."
And such an exceptional night would mean they would need five, since a Democratic shift in the landscape would take Lieberman from the Senate to the White House.
"We picked one up in Georgia, but we might lose one in Connecticut. This is very discouraging to us," grumbled Marvin Cox, chairman of the Democratic town committee in the hamlet of Chaplin, Conn., who is preparing a resolution to ask Lieberman to drop his Senate seat before the election so another Democrat can run in his place.
Republicans, of course, believe the point is moot, saying they are confident that Texas Gov. George W. Bush will be in the White House next year.
"I believe that Joe Lieberman is wise to keep his Senate seat," said Connecticut Republican Rep. Christopher Shays, "because ultimately he will be re-elected to the Senate, and he will not be the vice president."
And one seat may not matter much if the Democrats cannot mount more effective campaigns than the ones they have been waging against several vulnerable Republicans in the Senate.
Democrats entered this election season encouraged by a strong wave of campaign contributions and seemingly weak voter support for several Republican senators.
Between Jan. 1 last year and June 30 this year, contributions to the Democratic Senatorial Campaign Committee soared 135 percent over the same period four years ago, to $ 51.4 million, according to reports filed at the Federal Election Commission.
That nearly neutralized the Republicans' traditional cash advantage. The National Republican Senatorial Committee raised $56.2 million between Jan. 1 last year and June 30 this year.
But the Republican majority has benefited greatly from peace and prosperity - an environment that favors incumbents regardless of party - and from a calendar that has left Democrats battling each other in primaries two months before the general election.
"Certainly the Democrats have not advanced their cause at all in the last five months," said National Republican Senatorial Committee spokesman Stuart Roy said. "The primary season has really helped us."
Ugly Democratic primary fights continue in Minnesota, Washington, Rhode Island and Vermont - four states with potentially vulnerable Republican incumbents.
"It's an incumbent protection plan," Jim Jordan, political director of the Democratic Senatorial Campaign Committee, said of the calendar, which is determined at the state level. "It's out of our hands."
Democrats are favorites in two states, Florida and Delaware, according to Duffy.
Democratic Insurance Commissioner Bill Nelson is expected to beat Rep. Bill McCollum, one of the House prosecutors in President Clinton's impeachment trial, to win the Republican seat opened up by the retirement of Florida Sen. Connie Mack.
Delaware's popular Democratic governor, Thomas R. Carper, is slightly favored to defeat that state's also well-liked but aging Republican senator, William V. Roth Jr.
Minnesota's freshman Sen. Rod Grams, a former television anchorman whose conservative politics appear out of joint in his Democratic state, was once considered the most vulnerable of Republicans. But he won't even know who his Democratic opponent is until Sept. 12, eight weeks before the election.
Even so, Grams remains endangered. The two leading Democrats in the primary fight are Michael Ciresi, a wealthy trial lawyer, and former state auditor Mark Dayton, heir to the Dayton Hudson retail fortune. Either could snow Grams under with money, though both carry political liabilities.
In Michigan, freshman Republican Sen. Spencer Abraham remains relatively unknown in his state, and polls have not given him the support of more than 43 percent of the state's voters. He could also be hurt by the surging Gore campaign in Michigan and a ballot initiative on private school vouchers that should bring pro-Democratic teachers union members to the polls to oppose it.
"Neither party can claim [Michigan] as their state," Roy said.
But Abraham's Democratic opponent, Rep. Deborah Ann Stabenow, has even less name recognition and far less money. In a poll early last month, she had 39 percent of the vote.
Missouri offers another contest that will be close, pitting conservative Republican Sen. John Ashcroft against the state's current Democratic governor, Mel Carnahan. A poll last week found the race a dead heat, and a sharp swing in the Show Me state away from Bush toward Gore could help the challenger.
The Cook Report predicted that out of Minnesota, Missouri and Michigan, the Democrats would pick up one seat. And another might come their way in some wild-card races.
Their best prospects appear to be in Montana, where a stumbling Republican Sen. Conrad Burns faces an energetic insurgent rancher, Brian Schweitzer, and in Washington, where Republican Sen. Slade Gorton - never wildly popular in his state - is likely to face a little known but well-financed high-technology executive and former congresswoman, Maria Cantwell. A Washington state poll last week had Gorton and Cantwell dead even.
Two more potentially vulnerable Republican senators, Rick Santorum of Pennsylvania and John H. Chafee of Rhode Island, have been blessed by their opposition.
In Pennsylvania, Democratic Rep. Ron Klink has made some fundamental missteps, such as failing to get the list of campaign donors from the Democrats he vanquished in the primary. Now, he has too little money to compete in a very expensive race that will be saturated with advertising from the presidential campaign, Duffy said.
Rhode Island is a rock-solid Democratic stronghold. But Chafee's two Democratic opponents, Rep. Robert A. Weygand and former Lt. Gov. Richard Licht, have so bloodied each other in their still undecided primary contest that the winner may have little support left for the general election campaign.
Even if the tide turns Democratic in these Republican races, the Democrats must defend their own vulnerable open seats in New York, New Jersey and Nevada, and beat back a tough challenge to Democratic Sen. Charles S. Robb in Virginia.
The Nevada seat being vacated by retiring Sen. Richard H. Bryan will almost certainly go to former Republican Rep. John Ensign, who lost his Senate campaign two years ago by 428 votes. In Virginia, Robb is slightly behind in polls that favor popular former GOP Gov. George F. Allen.
First lady Hillary Rodham Clinton, the Democratic nominee in New York, and New Jersey Democrat John Corzine hold small leads in their races. But Clinton's opponent, Rep. Rick A. Lazio, is catching up in the money race and the polls. Corzine, a former Wall Street executive, is known more for his money than his political record, which could prove to be a liability in his contest against Rep. Bob Franks.
All this uncertainty has simply focused more attention on Lieberman's Senate seat.
Last week, John Orman, a professor at Connecticut's Fairfield University, complained to the state's election enforcement commission that Lieberman's re-election campaign was "frivolous" and "irresponsibly self-indulgent" and urged that he be removed from the Senate ballot. The appeal was rejected, but it revealed a simmering discontent in the state.
Late last month, the Democratic town committee in the northeastern Connecticut village of Hampton unanimously adopted a resolution asking Lieberman to drop his re-election bid. If he did, the state's well-known attorney general, Richard Blumenthal, could step in to face Lieberman's little-known Republican challenger, Waterbury Mayor Philip Giordano, who is barely a blip in the polls.
Under Connecticut law, Lieberman has until Oct. 27 to jump out of the Senate race.
"We have a popular attorney general in this state who could probably take the seat without a campaign," said Thomas Gaines, the member of the Hampton council who drafted the resolution.
A senior Democratic Senate strategist, speaking on condition of anonymity, predicted that Lieberman would drop out if Gore has pulled significantly ahead of Bush by Oct. 27.
But for now, most Democrats are saying they want Lieberman to stay on the ballot both as a vice presidential candidate and a Senate candidate. Even Blumenthal reluctantly agreed Lieberman should stay on that course.
"I would be very interested in becoming a candidate," he conceded, "but I think the prospects of his changing a correct and strongly supported decision are slim to none."