MOUNT LAUREL — A day after an election night standoff, Democrat Andy Kim claimed victory to a seat held by U.S. Rep. Tom MacArthur after thousands of mail ballots tallied by Burlington County election officials gave him the lead in New Jersey's hotly contested 3rd District congressional race.

"With the vast majority of votes now in and counted and based on the numbers we saw from Burlington County today, we have built a substantial lead," he told supporters crowded into his headquarters in an office park in Mount Laurel Wednesday night. "I am proud to announce that we have won this hard-fought race."

MacArthur, however, did not concede, issuing a statement saying, "This has been a hard fought campaign and like Andy Kim, I'm ready to see it come to an end." He said he would await the final count, including 7,000 votes he said were still outstanding.

The additional votes counted Wednesday from Burlington, the Democratic half of a district that also includes the Republican-leaning Ocean County, gave Kim a 2,600 vote lead out of about 295,00 votes tallied in the district.

The new vote total allowed Kim to overtake the 2,300 vote lead McArthur took away on Election Night, when the two-term Republican congressman left his Toms River headquarters without addressing supporters gathered there, and both campaigns called the race too close to call.

Kim, for his part, took the stage shortly after 12:30 a.m. Wednesday at his Mount Laurel hotel gathering and told supporters, "We can win this thing."

Even before the remaining mail ballots were added to the results, officials at the Democratic Congressional Campaign Committee said they were confident that the final results would lift Kim over MacArthur, who encountered a surge of suburban activism and an electorate's deep anxiety over health care in his quest for a third term.

The race featured months of negative advertising in two media markets, and millions spent by SuperPACs.

MacArthur, 58, a former insurance executive, was the only New Jersey congressman to vote in favor of the tax bill that capped deductions on state and local taxes, and helped craft an amendment that rescued the ultimately failed attempt to repeal the Affordable Care Act. Though he stressed his bipartisan credentials, he was the New Jersey Congressman most closely aligned with President Trump's agenda.

Kim, 36, worked as a national security advisor for the Obama administration, and moved back to the Marlton area, where he was raised, to run for office.

New totals showed Kim with 101,903 votes in Burlington County, an increase of 14,487 votes over previous totals.

MacArthur's totals also increased, but by 9,550, to 69,090 in Burlington County.

Burlington County Clerk Tim Tyler, who has held the elective office for 10 years, said Kim's 32,000 vote margin in the county -- mirroring MacArthur's advantage in Ocean County -- was significant. Though several thousand provisional votes still remained to be examined and then counted if found legitimate, Tyler said he would expect them to mirror the overall margin.

He also said the fact that poll workers failed to turn in all of the voting cartridges in five districts on Election Night should have little impact on the total tallies. Based on past experience, he said, some of the cartridges may be blank if some of the voting machines in a district were not used. Cartridges were not turned in by districts in Riverside, Mansfield, Pemberton, Evesham and Chesterfield.

Joseph Dugan, the chairman of the Burlington County Board of Elections, said the county received 26,000 mail-in ballots this year, maybe twice as many as last year and more than in 2016, a presidential election year. He said poll workers counted 20,000 of them on Election Day, from 7 a.m. to 10 p.m., and resumed counting the remaining 6,000 on Wednesday.

Election workers on Wednesday were busy removing them from envelopes and putting them in piles behind closed doors in a three-story county office building across from the courthouse. Dugan said their next chore would be to break the plastic seal on the canvas bags that contained the paper provisional ballots delivered by each voting district after polls closed. Next, they would turn them over to the superintendent of elections, on the second floor, so that he could match them up with voting registration records and see if the ballots are valid, Dugan said.

On Wednesday afternoon, lawyers and party officials milled around and waited for the tallies to be announced.

An official winner might not be declared for days, as officials in both Burlington and Ocean counties also have to process provisional ballots, which are given to voters who show up at polls despite having received a mail ballot, among other reasons. There were several thousand of those outstanding between the two counties.

In Ocean County, vote by mail ballots were included in the Tuesday night totals, county officials said.

The race broke the way pollsters had predicted: an essential standoff between the two halves of the district: Ocean County to the east, with its many retirees and solid Republican base, and Burlington County across the river from Philadelphia, where Democratic registration had increased significantly in the last two decades, said Benjamin Dworkin, a political science professor at Rowan University.

The surge in Democratic voting in Burlington County on Tuesday was enough for voters to take over long-held Republican county offices, including the clerk and the Board of Chosen Freeholders. Places like Willingboro, where voting is typically light in midterms, saw heavier turnout and even waits at some polling places.

In addition, newly organized progressive groups like Action Together New Jersey, and its Burlington and Ocean county spinoffs, spearheaded extensive vote by mail operations in the last year.

Uyen Khuong, who founded the Action Together group after the 2016 election, said Wednesday the group's thousands of volunteers used a database (and their own stamps) to target nearly 300,000 voters state-wide in 16 week. This "post-it posse" sent out applications for vote by mail ballots, along with a post-it note urging voters to consider voting by mail, which New Jersey allows for any reason.

In the Third Congressional District, Khuong said, 82,000 applications were sent out.

She said she alerted state and county officials ahead of time to print out extra ballots, telling them, "I'm going to do a huge push to change the voting habits of New Jersey voters."

The number of vote by mail ballots also increased significantly this year because of a new law that required all voters who had voted by mail in 2016 to receive a mail ballot for this election.

Ocean County Republican Chairman George Gilmore blamed the law, and Gov. Murphy, for creating confusion and requiring voters to use provisional ballots when they arrived at the polls and were told they were a "vote by mail" voter. There were several thousand of those provisional ballots in each County, according to county officials.

Democrats picked up three other Republican-held house seats in New Jersey Tuesday night: State Sen. Jeff Van Drew won in New Jersey's second district; Navy pilot and former federal prosecutor Mikie Sherrill won in the 11th, and in the 7th district, Tom Malinowski unseated incumbent Rep. Leonard Lance.

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