"As we certainly know in California, that election night a lot of times is only halftime. There's going to be votes coming in the days and weeks afterwards," Cox said.
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In 2018, Valadao led by around 4,400 votes on election night and the Associated Press declared him the winner - only to retract the call four weeks later once mail-in and provisional ballots were counted.
This time around, the race remains as tight as ever with Valadao currently leading by a razor-thin margin.
As of Wednesday morning, Valadao was up on Cox by about 3,000 votes with 100% of precincts reporting.
"Seeing the numbers come in early made us feel good, but we learned from last year. We're not going to take anything for granted and we're watching every ballot get counted. And making sure we're doing this as legit as possible," Valadao said.
The 21st District includes all of Kings County and portions of Fresno, Kern, and Tulare counties - among registered voters 44% are Democrats and 27% are Republicans.
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Despite the odds against him, Valadao - a dairy farmer from Hanford - has remained a favorite among voters since 2012 before getting edged out by Cox two years ago.
Political analyst Mark Kepler predicts this race will go down to the wire once again.
"The district seems to be ready-made for a Democrat but Valadao has won most of the races he's run and he's very competitive, obviously, so this is going to be one of those nail biters," Kepler said.
With another close race in the rematch, both candidates remain optimistic one day after polls closed.
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"We're going to wait until every vote is counted, every voice is heard and then we'll be satisfied with the result," Cox said.
Said Valadao:
"I'm proud of the work that I've done but I'm also super proud of my team and volunteers have done. I'm excited what we're going to see over the next few days, even possibly weeks, but we feel good about this race."
With vote by mail and provisional ballots still to be counted, it may take weeks before we know who wins the 21st Congressional seat.
RELATED: 2020 CA election results by county, propositions, electoral college votes