WASHINGTON - 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 Obama can ill afford to lose. Republicans have repeatedly pushed their own versions of legislation through the House in the last 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 G. 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.

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.

In Missouri, a three-way Republican primary is on the horizon, but already Democratic Sen. Claire McCaskill is rated among the most endangered incumbents of either party.

In Montana, Democratic Sen. Joe Tester has been defending his seat against a challenge from Rep. Dennis Rehberg in a race that is expected to remain close through Election Day.

Democrats' chances got a lift in Maine with Sen. Olympia Snowe's decision to retire, although they still have virtually no chance of winning the seat outright.