Buckle In: California's Primary Results Could Take A While

(CNN) -- California's high-stakes primary takes place Tuesday, but it could be a much longer wait compared to other states before the winners are known.

Democrats and Republicans say they are bracing to wait days or even weeks before knowing the winners of some of the most closely contested contests, particularly House primaries.

The first votes tallied will be vote-by-mail ballots counted in the 8 p.m. Pacific time hour. Election day results will begin flowing into the California secretary of state's office in the hours that follow.

However, there will be many -- perhaps millions -- more votes to count as they arrive in the mail at county elections offices on Wednesday, Thursday and Friday.

To be counted, vote-by-mail ballots only have to be postmarked by Tuesday. They can arrive as much as three days later.

Counties are due to report how many mail ballots are outstanding to the secretary of state's office by Thursday. That means the state isn't even certain how many ballots remain unaccounted for until later in the week.

California's jungle primary system means the top two finishers, regardless of party, advance to the general election.

Because more Democratic candidates than GOP candidates are running in several competitive districts, Democrats are worried that the party could get locked out of the top two in as many as three districts they otherwise believe are winnable in November's midterm elections.

How long can it take to finalize California's results?

It took 20 days after the 2016 general election for The Associated Press to declare Republican Rep. Darrell Issa the winner in a tight race in southern California's 49th District.

The delay Democrats might remember best, though, came in the 2016 matchup between Hillary Clinton and Vermont Sen. Bernie Sanders. With 700,000 provisional ballots slowing the counting, it took a full month for the state to finish tallying its presidential primary results.