ASHINGTON - Buoyed by polls showing surprisingly tight races in several critical states, Democrats are now in striking distance of taking back control of the Senate, a prospect that appeared extremely unlikely just a few months ago.
In seven contested states, Democrats are either leading in the polls or are running neck-and-neck against their Republican opponents, six of whom are incumbents. The Democrats' most vulnerable senator, Charles S. Robb of Virginia, has narrowed his deficit, and is now all but even with his challenger, George Allen.
While the Democrats are likely to lose a seat in Nevada, they could still take a slim majority in the US Senate, where Republicans now hold a 54-46 advantage.
The high stakes, particularly in the context of closely fought contests for control of the House of Representatives, have Senate candidates spending record sums in increasingly nasty campaigns.
''This has all happened in the last few days ... I'd say the Senate is very much up for grabs,'' said Steven Schier, a political science professor at Carleton College in Minnesota.
The Democrats are now ''dead man walking,'' after being written off as buried, said Larry Harris of the nonpartisan Mason-Dixon polling operation.
From Washington to Florida, the money is flowing and the mud is flying, as candidates struggle to eke out percentage points.
Republicans have an inherent disadvantage in this set of elections, because 19 of their incumbents are being challenged, against 15 for the the Democrats. Republicans also have more vulnerable incumbents; in six races, the sitting Republican is facing a tough challenge. Only one Democratic incumbent, Robb, is considered at risk of losing his job.
Still, the Democrats must show a net gain at least four seats. The equation could become more complicated because Joseph I. Lieberman has decided to seek reelection to his Connecticut Senate chair even while running for vice president.
The Republican governor of Connecticut would name Lieberman's successor, presumably another Republican, if voters send Al Gore and Lieberman to the White House, and if Lieberman wins reelection to the Senate.
''There's a lot in play,'' said Stuart Roy, spokesman for the National Republican Senatorial Committtee. ''It's a trifecta election: the White House, the Senate, and the House are arguably within reach of either party.''
''We can do it,'' said David DiMartini, spokesman for the Democratic Senatorial Campaign Committee. ''We just need to get all the breaks.''
There is no single theme to the strongly contested races, nor are challengers benefiting from an anti-incumbent sentiment. Rather, analysts say, the close races reflect the dynamics of the presidential race, in which voters appear almost evenly divided between Gore and George W. Bush.
One of the hottest races is the contest in Michigan, where Representative Debbie Stabenow, a Democrat, is challenging incumbent Senator Spencer Abraham. Once behind in double digits, Stabenow has pulled slightly ahead in at least one poll, and running even against Abraham in other surveys.
The campaign has taken a decidedly negative turn recently. Stabenow has accused Abraham of catering to ''special interests'' that she says funds his campaign. Abraham has skewered Stabenow as ''Liberal Debbie,'' whose priorities are out of step with Michigan.
Both candidates have posted Web sites dedicated to discrediting their opponents; Abraham's shows a picture of Stabenow morphing into the face of Hillary Rodham Clinton, and raising the question, ''Who is more liberal, Debbie or Hillary?''
''The last two weeks, everything gets ugly,'' said Harris of Mason-Dixon. In Virginia, Robb's campaign warns women that Allen, a former governor and son of the former Washington Redskins football coach, opposes their right to have abortions. Allen, meanwhile, is running ads accusing Robb of depriving Virginia of transportation funds.
Most of the threatened Republicans are running as moderates, seeking to win over malleable Democrats, Harris said. Bill McCollum of Florida, for example, has adopted the Gore phrase ''lockbox'' to demonstrate his concern to protect Social Security. McCollum, a floor manager for the impeachment effort against President Clinton, has also come out in favor of a hate-crimes bill, to the chagrin of some of his colleagues.
The tightening of the races largely reflects a late spending spree by Democrats, said Carleton College's Schier. Several challengers, including Stabenow, held their cash for late ad blitzes.
Senate candidates are raising and spending money at unprecedented levels. A New Jersey Democrat, Jon Corzine, has spent $54 million in his race against the Republican incumbent, Representative Bob Franks. The amount is almost $10 million more than Gore has raised, independent of federal financing.
The New York Senate race between Clinton and her Republican rival, Representative Rick Lazio, is the most expensive so far; the two have raised almost $80 million. The New Jersey candidates have raised a combined $71 million, and the Minnesota Senate contenders have a $23.6 million kitty.
''We made a conscious decision to reserve what we had for the last week of the campaign, and we're matching our opponent dollar for dollar, ad for ad,'' said Robert Gibbs, Stabenow's communications director. ''Now, you can't turn on the TV without seeing wall-to-wall commercials.''
