WASHINGTON – Republicans’ clear shot at winning control of the Senate is attracting tens of millions of dollars from GOP-allied outside groups eager to spend on a surer bet than a White House race with a resurgent President Obama and an unsettled GOP field.

Republicans need to capture four Democratic seats to grab the majority and Democrats have all but conceded one — Nebraska — where Sen. Ben Nelson decided against a third-term bid in the heavily GOP state.

Control of the Senate will hinge on tight races in Massachusetts and Nevada, where Democrats see their best chances of unseating two of the newest Republican senators, Scott Brown and Dean Heller; Montana and Missouri, where Democrats Jon Tester and Claire McCaskill won narrowly in 2006, and open Democratic seats in Virginia and Wisconsin, according to Republicans, Democrats, campaign consultants and lobbyists.

By the numbers, the odds heavily favor the GOP; Democrats are defending 23 seats, including six open seats and one independent, to the Republicans’ 10.

“Republicans are well-positioned to pick up Senate seats, and the message from our candidates will be a simple one — if you want a check-and-balance on the Obama agenda and to restore fiscally-responsible, pro- jobs policies in Washington then you can start with a Republican-controlled Senate,” Sen. John Cornyn, R-Texas, chairman of the National Republican Senatorial Committee, said in a statement.

But Democrats are expressing optimism about their prospects of keeping the majority. Obama’s steadily improving standing with the electorate, signs of a healthier economy and housing market and the lack of clarity in the highly divisive GOP presidential field are energizing Democrats. The current Senate breakdown is 51 Democrats, 47 Republicans and two independents who caucus with the Democrats.

“I know when I took this job a year ago, most people felt it was really mission impossible, that the economic climate, the number of folks that we had up, was going to make it virtually impossible,” said Sen. Patty Murray, D-Wash., chairwoman of the Democratic Senatorial Campaign Committee. “I think that has changed dramatically in a year.”

Obama’s deep-pocketed campaign and its expected spending in battleground states is expected to lift down-ballot Democratic candidates.