SACRAMENTO -- Tens of thousands of ballots from one of California's most populous counties will go uncounted on Election Night, potentially complicating the effort to call winners for the presidential primaries and propositions.

Sacramento County's elections chief said that widespread problems with the county's paper ballot machines have forced her to halt the practice of scanning ballots as they come into the 548 polling places in the state capital.

Instead, officials will revert to counting votes the old way: After polls close, they will drive ballots to a central office and manually feed them into one of seven machines.

In a best-case scenario, the county will not begin counting ballots until about 9 p.m., an hour after polls close.

With its seven machines, Sacramento County will be able to count a maximum of about 49,000 ballots an hour. Elections officials are making no promises to call races before 9 a.m. the next day.

"We had 100 machines or so that we were having problems with, and it was just uncomfortable," Sacramento County registrar of voters Jill LaVine said. "After much research and no solution, I didn't want to send out any of the machines. I just pulled the whole plan."

She said that during standard, pre-election testing, the Elections Systems & Software machines that the county has used for six years inexplicably began rejecting several versions of the Feb. 5 paper ballot.

She said she consulted with Secretary of State Debra Bowen, whose office backed the contingency plan.

Sacramento has nearly 600,000 voters registered for next month's primary, including 257,000 Democrats, 202,000 Republicans and 113,000 who declined to state a party affiliation.

A third of those vote by absentee ballot.