Republicans hold 55 seats in the Senate, to 44 for Democrats, with one independent. Republicans have reason to worry about seats in Ohio, Pennsylvania, Rhode Island, Montana and Missouri, and conceivably Tennessee and Arizona. If Sen. Trent Lott of Mississippi does not run again, even that seat could be in play, strategists for both parties say.

But this is not an easy road for the Democrats. Not only would they have to win races in the South, hardly friendly territory, they would need to hold off potentially strong Republican challenges to Democratic seats in New Jersey, Minnesota and Maryland.

"It's not likely, but I can see the math," said Jennifer Duffy, an analyst for The Cook Political Report, a nonpartisan newsletter.

What to watch?

Start with Arizona and Tennessee, the toughest of the bunch. If Republican senators there start to falter, Democrats could indeed be poised to take back the Senate. In Arizona, Sen. Jon Kyl, after winning a second term without Democratic opposition in 2000, faces a well-financed challenge from Jim Pederson, a former Democratic Party leader. In Tennessee, Sen. Bill Frist's exit has set up a potential three-way Republican primary, while the Democratic candidate will be a well-known moderate congressman, Harold E. Ford Jr.

From there, look to Montana to see if Republican ethical problems might help Democrats this year. The state's Republican senator, Conrad Burns, had to return a $150,000 campaign contribution from Jack Abramoff, the indicted Washington lobbyist at the center of a congressional corruption investigation, and Democrats are viewing him as increasingly vulnerable.

For one sign of whether the war in Iraq is playing to the Democrats advantage, look to Ohio, where Paul Hackett, an Iraq war veteran who lost a bid for Congress last year, is seeking the Democratic nomination to challenge Sen. Mike DeWine, a Republican. Republicans are having all sorts of problems in Ohio this year.

On the other hand, if Democrats seem to be struggling to take out Republican incumbents in Rhode Island and Pennsylvania come the fall — both Rick Santorum of Pennsylvania and Lincoln Chafee of Rhode Island seem particularly vulnerable right now — that will not bode well for the party. — Adam Nagourney

BUSINESS — WARMING UP UNDER THE SOLAR PANEL

Until recently, the alternative energy industry was decidedly alternative. It conjured up images of off-the-grid bohemians heating their cabins with wood-burning stoves, and eccentric tinkerers tricking out cars to run on vegetable oil.