The departure of Jesse Helms and Strom Thurmond, two of the South's best-known and longest-serving U.S. senators, highlights the difficulties the Republicans face as they try to overturn the Democrats' razor-thin margin and regain control of the Senate in 2002.

The party is defending 20 seats, including the only two open ones, to the Democrats' 14. Republicans expect to counter that apparent disadvantage by exploiting favorable geography, putting up strong challengers to Democratic incumbent senators in states won by President Bush in 2000.

The most important factors in the next Senate election probably won't be the number of seats each party is defending, but history and the economy.

"Right now the playing field is about even, and everything depends on the Bush administration," said conservative analyst Bill Kristol. "The best news for Republicans is a popular Bush and a recovering economy. ... The nightmare scenario is Bush in trouble and a faltering economy."

The departure of Helms, R-N.C., among the most powerful conservative voices in the country, is one of several developments in the coming days that could shape the landscape for Senate elections in 2002. The Democrats have a precarious 50-49-1 advantage after Vermont Sen. Jim Jeffords switched from the GOP to independent in June. Helms was reported ready to announce in a Wednesday night broadcast his decision to retire at the end of his current fifth term.

Another piece of the puzzle fell into place when New Hampshire's Democratic Gov. Jeanne Shaheen files papers as expected Wednesday to form an exploratory committee looking at the Senate seat now held by Republican Bob Smith. Rep. John Sununu is widely expected to challenge Smith within the GOP.

"We are waiting for some big decisions that will finish the picture and show how competitive this is going to be," said Jennifer Duffy, a Senate election analyst for the Cook Report.

Other decisions expected soon:

--South Dakota's lone U.S. representative, John Thune, a Republican, will say this fall whether he'll challenge the incumbent Democratic senator, Tim Johnson.

--Oregon's Democratic Gov. John Kitzhaber must announce whether he'll take on incumbent GOP Sen. Gordon Smith, expected early next month.

--Tennessee Sen. Fred Thompson, a Republican, hasn't announced whether he'll seek re-election.

Democrats are targeting New Hampshire, Oregon and Arkansas for possible pickups, although they concede gaining one of the two open seats in the Carolinas could be tough. Thurmond, who is from South Carolina, announced during the 1996 campaign that it would be his last.

Republicans may have more turf to defend, but they note many of their seats are in reliably GOP territory in the West and South.

Democrats are defending seats in states President Bush carried in 2000 -- Montana, South Dakota, Missouri, Georgia, Louisiana -- and in states Vice President Al Gore narrowly carried, such as Iowa and Minnesota.

Sen. Bill Frist of Tennessee, chairman of the National Republican Senatorial Committee, has said Republicans will run "with a lot of discipline, with a very simple message of retaking the United States Senate."

The Bush administration, which considers reclaiming the Senate a top priority, is taking an active role in recruiting Senate candidates in Minnesota, Missouri, South Dakota and other states. "These races are going to be run on local issues," said Dan Allen, a spokesman for the GOP senatorial committee. "The president was popular in a lot of the states we're going to have to contest."

The aggressive posture of the Republicans isn't intimidating for Sen. Patty Murray of Washington, chairwoman of the Democratic Senatorial Campaign Committee.

"The country likes it when there is a debate on issues and balance in government," Murray said. "They like having a Democratic majority in the Senate." And she said the economy's troubles pose "an extreme potential danger for the Republicans."

Committee spokeswoman Tovah Ravits said Democrats have been winning Senate races for many years in states that Bush won last year.

The news about Helms brought praise for his years of service, but many Republicans were just as enthusiastic about efforts to draft Elizabeth Dole as a North Carolina candidate.

Several senior Republicans said she would bring star power to the race. Others, including some in the Bush camp, said it might be better if the party turned to a younger candidate with closer ties to the state. Dole wasn't considered a stellar presidential candidate or fund-raiser by GOP analysts.

The evolving picture in the Senate has yet to give either side a clear advantage, says political analyst Thomas Mann of the Brookings Institution, who said:

"The seats at risk balance out."