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. It's confounding early forecasts that Democratic rule is inevitably coming to an end.

Adding to the uncertainty, tea party-backed challengers are on primary ballots against establishment candidates in New Mexico and Texas in coming weeks. That's 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 legislation through the House in the past year, only to have it blocked by the Democratic Senate.