By DAVID ESPO
AP Special Correspondent
WASHINGTON (AP) - Divisive Republican primaries, an out-of-nowhere GOP retirement in Maine and an unexpectedly competitive race in North Dakota add up to an unpredictable battle for control of the Senate this fall, confounding early forecasts that an era of Democratic rule was inevitably coming to an end.
Adding to the uncertainty, tea party-backed challengers are on the primary ballot against establishment candidates in New Mexico and Texas in the coming weeks, a continuation of an internal Republican struggle that Democrats hope will aid them as it did in 2010.
With the support of two independents, Democrats now hold an effective 53-47 majority in the Senate, control that they and President Barack Obama can ill afford to lose. Republicans have repeatedly pushed their own versions of legislation through the House in the past year, only to have it blocked or altered by the Democratic Senate.
Democrats are defending 23 of the 33 Senate seats on the ballot this fall, including the two held by independents. Republicans must gain four to be assured of a majority when the new Congress convenes in 2013.
Republicans claim Nebraska, North Dakota, Missouri and Montana as the states where they have the best chance to pick up seats, followed by Wisconsin, New Mexico and, possibly, Ohio.
Democrats point to Maine, Massachusetts, Nevada and perhaps Indiana, where veteran Sen. Richard Lugar fell in a primary to a tea party-backed challenger, as opportunities to offset any losses they suffer elsewhere. They also claim hopes for Arizona, which Obama's campaign hopes to make competitive in his race against Mitt Romney.
"I would say we still have a great opportunity to win a majority," said Rob Jesmer, the executive director of the Senate Republican campaign organization. He said his party is defending only three seriously contested seats it holds and is mounting strong challenges in 10 or 12. His bottom line: "We feel good about our chances, but it is going to be close."
Even that is an acknowledgment of a shift that has taken place in the political landscape since the two parties began evaluating their chances for 2012 more than a year ago.
" I just remember when I took this on a year and a half ago, there wasn't anyone who said this was easy or we were going to get the majority or we even had a chance," said Sen. Patty Murray, the head of the Democratic counterpart organization. "The map has changed dramatically" since then, she said.
Still unknown is the full impact of outside groups, including the newly formed super PACs that operate under rules that allow them to accept donations of unlimited size.
For example, conservative organizations already have spent millions of dollars on television advertising against Democratic Sen. Sherrod Brown of Ohio in hopes of making him vulnerable to a challenge, $6.5 million by his campaign's own count.
While outside groups have recently pumped in an estimated $700,000 to support Brown, a spokesman said Wednesday his lead in the race as measured in public polling has been cut roughly in half. The first-term incumbent recently began advertising on his own.
The list of competitive races underscores the impact that retirements, candidate recruitment and primaries have had on the fortunes of the two parties since the 2010 elections.
Democratic Sen. Ben Nelson's decision to retire in Nebraska opened up an early opportunity for Republicans.
Democrats countered by recruiting Bob Kerrey, a Medal of Honor winner, former governor and former senator who has more recently been a university president in New York _ and boasted they had at least made the race competitive.
But little-known state Sen. Deb Fischer emerged victorious Tuesday night in a three-way Republican primary after winning an endorsement from Sarah Palin and benefiting from a $200,000 ad campaign backed by TD Ameritrade founder and Chicago Cubs co-owner Joe Ricketts.
Privately, Republicans and Democrats both said Wednesday that her win had probably made Kerrey's comeback more difficult.
North Dakota looked like a sure thing for Republicans when Democratic Sen. Kent Conrad announced plans to retire.
But Democratic challenger Heidi Heitkamp, a former attorney general, is credited with running a strong campaign that even Republicans concede has made her race against Rep. Rick Berg a competitive one _ even though the state is expected to vote heavily for Romney this fall. At a minimum, the GOP and allied groups are likely to be forced to spend money on television advertising that once seemed unnecessary.