WASHINGTON (AP) — All the pieces are in place for a re-election bid, but Nebraska Sen. Ben Nelson is wavering on whether he'll seek a third term in 2012 or retire — a decision that could have a big effect on Democrats' chances of holding their Senate majority.Nelson says he'll make a decision over the holiday season. In the meantime, he's piled up campaign cash, hired a campaign manager and watched his party spend more than $1 million on ads supporting him.Nelson, a conservative Democrat, has told friends and supporters he's not a lock for the race and expressed frustration with Congress' ability to pass meaningful legislation.Republicans need to net four seats in 2012 to take control of the Senate and will target the seat either way.