Republican hopes of expanding the party's Senate majority begin in Nebraska, where first-term Democrat Ben Nelson is bidding for reelection in a state President Bush won by a landslide.

But Nelson, a leader in putting together last month's bipartisan pact on judicial nominees, is proving that red-state Democrats can still win fans by sticking to the political center and acting as can-do problem solvers who put pragmatism above party.

Already known for breaking with his party's leaders by backing Bush's tax cuts and considering the administration's Social Security proposals, Nelson thrust himself into the center of the effort to avert a Senate meltdown over judges. Last week, he proudly told Nebraskans that he wants Congress to stay focused on highway construction, retirement security and other issues they care about. One detail Nelson routinely omitted did not surprise those who watch him closely.

"Nelson will never say he's a Democrat," said University of Nebraska political scientist John Hibbing. It's a smart strategy, he said, in a state where registered Republicans heavily outnumber Democrats but voters embrace an independent spirit reflected in their one-of-a-kind nonpartisan, unicameral legislature.

Triumphant visits such as the one Nelson enjoyed here and in Omaha are troubling to the GOP. If the party is to inch closer to a filibuster-proof Senate majority -- 60 votes -- campaign experts say, Republicans must step up their candidate-recruitment efforts and their critiques of Democrats in Nebraska, North Dakota, Florida and other states Bush carried.

Republicans stress that the election is 17 months away and Democrats face their own problems in several states. But Nelson's homestate visit suggests that centrist Democrats with discipline, campaign skills and luck can still generate considerable support in states their party long ago surrendered at the presidential level.

"I appreciate what you're doing on the judge thing. That was a work of genius," John Cutler effusively told Nelson when the senator toured Lincoln Benefit Life Co., where Cutler is a documents specialist.

Nelson's reception was just as warm at a luncheon hosted here that day by the Nebraska Chamber of Commerce -- about as Republican a group as can be imagined. "We owe you a debt of great gratitude" for helping pass a bill limiting class-action lawsuits, said the audience member called on to ask the first question.

Nelson, a former two-term governor who lost a Senate bid in 1996 and narrowly won an open seat in 2000, has tried, not always successfully, to forge bipartisan responses to federal-employee bargaining rights, tax policies and other issues. Last month, Nelson drew headlines and accolades when he helped craft the compromise on judges.

Nebraska editorial writers are lauding Nelson, and business groups are thanking him for his pro-business efforts. Republicans, meanwhile, wonder whether they are losing a chance for a Senate seat in the state that gave Bush his fourth-largest margin of victory last fall.

At every stop, Nelson tells Nebraskans of his credo: "I'll support the president when I can, oppose when I must. I'll always look for a compromise and solution when possible, and I won't obstruct." In a meeting with 40 Lincoln Benefit Life employees, who asked no hostile questions, Nelson vowed to prevent Republicans from defining him.

"I don't want to ban the Bible," he said. "I don't want to take away your guns. I'm not for gay marriage." As he described the political left and right in Congress, a woman in the front row held her hands two feet apart to symbolize the political center. "There's a lot of us right here," she declared. Nelson beamed.

Republicans hold 55 of the Senate's 100 seats, and a few gains in next year's elections would make it extremely difficult for Democratic leaders to sustain filibusters, an action that requires 41 votes.

As in recent elections, Democrats anxiously ponder the Senate's political math, which does not favor them. The more Senate races tend to reflect presidential outcomes, the stronger it makes the GOP in the Senate. For example, Bush won 31 states last year. If Republicans hold all the Senate seats from those states, they will command the chamber 62 to 38, even if they lose their eight members from states that Democratic presidential nominee John F. Kerry won last November.

Last fall, Republicans won all five southern seats from which Democrats retired, and Democrats are desperate to reelect their incumbents in tough states next year. Topping the GOP's target list are Nelson and Sen. Kent Conrad of North Dakota, where Bush took 63 percent of the vote last fall, only slightly lower than his 66 percent majority in Nebraska.

However, in both states the GOP faces recruiting problems. The only prominent Nebraska Republican to announce thus far is former attorney general Don Stenberg, a less-than-stellar campaigner who narrowly lost to Nelson in 2000. To the bitter disappointment of Senate recruiters, Rep. Tom Osborne (R) -- the former Nebraska Cornhuskers football coach still revered here -- is running for governor, even though the incumbent, Dave Heineman, who was sworn in as chief executive in January, is a fellow Republican.

Nelson would probably have been the underdog if Osborne or popular former governor Mike Johanns (R) had challenged him, Hibbing said, but now GOP recruiters "are at the second tier."

Moreover, GOP activists say, Bush has not helped their effort. In December, he appointed Johanns, then the governor, to be secretary of agriculture, depriving the party of its most promising Senate challenger. Then, on Feb. 4, Bush traveled to Omaha with Nelson and praised the senator's openness to White House proposals to restructure Social Security. He called Nelson "a man with whom I can work, a person who is willing to put partisanship aside to focus on what's right for America."

In North Dakota, Republicans have only one potential candidate who might oust Conrad, a 19-year Senate veteran who has won reelection easily, according to analysts there and in Washington. Gov. John Hoeven (R) has the skills and popularity to press Conrad hard, they say, but it is unclear whether he will run for the Senate in the middle of his second term.

Republicans also have recruitment concerns in Florida, which Bush carried 52 to 47 percent over Kerry. Many Republicans feel their best candidates are running for the open governor's seat rather than challenging Sen. Bill Nelson (D). Among those weighing a Senate bid is Rep. Katherine Harris, who gained national attention while overseeing the chaotic state ballot recount that helped Bush win over Al Gore in 2000. Harris is so popular among grateful GOP die-hards that she might win the Senate GOP primary, analysts say. But her general election prospects are less certain. "She's still really polarizing," said Jennifer Duffy, who tracks Senate races for the nonpartisan Cook Political Report.

Democrats, who are defending 18 seats to the GOP's 15, see their best hopes for Senate gains in Pennsylvania and Rhode Island. Bush lost those states in 2000 and last year. Sen. Rick Santorum (R-Pa.), an outspoken conservative, expects a strong challenge from state Treasurer Robert P. Casey Jr. (D).

In Rhode Island, Sen. Lincoln D. Chafee (R) is more liberal than several Democratic senators, but that is not good enough, Democratic strategists say.

Democrats jockeying to challenge Chafee include Secretary of State Matthew A. Brown and former state attorney general Sheldon Whitehouse.

Among the four open races in next year's Senate elections, Republicans say they are confident of keeping the Tennessee seat being vacated by Majority Leader Bill Frist. Maryland, where Paul S. Sarbanes (D) is retiring, leans strongly Democratic, but the GOP hopes Lt. Gov. Michael S. Steele will make a strong run. Democrats are favored in Vermont, where independent Sen. James M. Jeffords is stepping down. Republicans will make a strong push to replace Sen. Mark Dayton (D) in Minnesota. Rep. Mark Kennedy has the GOP field largely to himself, while several Democrats are weighing campaigns.

In Washington state, Sen. Maria Cantwell (D) has shaky approval ratings, but the strongest potential GOP challenger -- Dino Rossi -- is fixated on claiming the disputed 2004 governor's race, insiders say.

Republicans say they have outside chances of ousting Democratic Sens. Robert C. Byrd (W.Va.) and Debbie Stabenow (Mich.) if they recruit the right challengers. Likewise, Democrats say they see vulnerability in Republican Sens. Mike DeWine (Ohio), Conrad Burns (Mont.) and James M. Talent (Mo.).

Sen. Ben Nelson, left, is one of 18 Democrats whose seats are on the line next year. Republicans are defending 15.