LOS ANGELES (Reuters) - California’s pro-Russia congressman, Dana Rohrabacher, is one of four Republican incumbents in the state whose seats are still in danger of falling to Democratic opponents in the U.S. House of Representatives as America’s most populous state slowly counts its votes.

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The outcome of those races - and how much they add to the Democrats’ majority in the newly constituted House - may be uncertain for weeks to come because of mail-in and provisional balloting systems that California introduced in recent years to boost voter participation.

According to the latest projections by media outlets and data provider DDHQ, Democrats have achieved a net gain of 32 House seats nationwide.

California election results tallied as of Wednesday do not account for millions of outstanding ballots still to be processed statewide. Many were mailed by voters to local registrar offices by Tuesday’s deadline but have yet to be delivered.

In Orange County alone, well over 400,0000 votes remained left to be counted, more than half of them mail-in ballots, according to data provided by the county registrar’s office on Wednesday. Tens of thousands of provisional ballots for voters who waited until election day to register also are outstanding.

State returns on Wednesday showed Rohrabacher, a 13-term congressman in coastal Orange County in Southern California, trailing his Democratic challenger, Harley Rouda, a real estate entrepreneur and former Republican, by fewer than 3,000 votes - or 49.3 percent to 50.7 percent.

Rouda, whose lead improved slightly on Thursday to 51 percent, versus 49 percent for Rohracher, estimated that 70,000 ballots remained to be counted in the district.

Rouda said he was “cautiously optimistic” that “our lead will hold and in all likelihood will increase” when all votes are in, but said he would hold off on any announcements for at least several days.

The Rohrabacher campaign did not immediately respond to requests for comment.

The Rohrabacher-Rouda race has stood out nationally in part because of the incumbent’s reputation for unabashedly pro-Russian views, widely seen as once putting him on a short-list for consideration as President Donald Trump’s secretary of state.

Rohrabacher’s seat is one of several held by a Republican in a California district that supported Democrat Hillary Clinton over Trump in the 2016 presidential election or was otherwise seen as vulnerable to flipping Democrat.

That race and three others in the state where Democrats were seeking to pick up a Republican seat were either still in play or, as was the cases in two districts, leaning so heavily toward Democrats that they had claimed victory.

However, Republican incumbents appeared to have garnered comfortable leads in at least three other House races in Southern California that Democrats had viewed as up for grabs.

Among them was Duncan Hunter, who has pleaded not guilty to felony corruption charges but managed to defeat Ammar Campa-Najjar, a former Obama administration aide of Mexican and Palestinian heritage.

No races will be certified for at least a month, the time election officials are given to complete their “official canvass” of all votes. They have until Dec. 7 to report final results to the California secretary of state, who then will have another week to officially declare winners.