Andy Vidak’s lead over incumbent Jim Costa in the nationally watched 20th Congressional District race shrank to just 145 votes Monday evening as Kern County updated its tally of counted ballots.

Democrat Costa claimed 621 more votes Monday than Vidak did in Kern County, chopping yet another chunk off of the 1,823-vote lead Vidak recorded on election night.

Republican Vidak, who’d lost ground in both Fresno and Kern counties on Friday, was able to keep a narrow grip on the lead in part due to results released by his home county, Kings, earlier in the day Monday.

Vidak picked up 118 more of Monday’s Kings County votes than Costa, at that point extending his lead to 766 votes. Vidak has been beating Costa handily there with nearly 70 percent of the vote, but Kings has far fewer voters in the 20th District than Fresno and Kern counties have together.

Kings County elections officials said they’ve completed their ballot count.

With most of Kern’s ballots now accounted for as well — with the exception of a few thousand challenged and provisional ballots county-wide — the ultimate outcome of the race will likely be decided by Costa’s home county of Fresno.

And that means Costa and Vidak will have to wait until at least Wednesday at 3 p.m. to find out their fate.

That’s when Fresno County will issue its next voting update, said Registrar of Voters Victor Salazar.

Fresno County vote-by-mail ballots, which trended strongly in Costa’s favor on Friday — he picked up 732 votes there — would need to turn sharply toward Vidak on Wednesday to overcome Monday’s results from Kern.

Vidak and Costa’s camps were non-committal about the race Monday.

“We can all sit here and speculate. What we really need to do is we need to wait until this thing is done,” said Vidak spokesman Rudy Mendoza. “We do expect the momentum to continue behind our campaign.”

Costa said from Fresno Monday that he remains optimistic.

“I'm feeling good. I'd rather have my numbers rather than his,” Costa said. “Kings County is done. I'm going to win Kern by a very substantial margin and I'm going to do the same in Fresno. We're going to wait 48 hours and hopefully we'll be able to bring this to a successful conclusion.”

Kern County elections chief Karen Rhea said all of the vote-by-mail ballots that haven’t been challenged, or damaged during mail delivery, were counted over the weekend and were included in the Friday update.

That included an estimated 4,725 ballots in the 20th.

But Kern’s role in this race may not yet be over.

Rhea said an additional 3,000 county-wide ballots are being closely reviewed to make sure they are valid.

And 7,102 provisional ballots — ballots where there was some question about the eligibility of a voter on Election Day — have yet to be counted.

How many 20th District votes are in those pools is unknown.