Longtime U.S. Rep. Eliot Engel of New York, who was first elected in 1988 and rose to become one of the most powerful Democrats in the House, was substantially trailing middle school principal Jamaal Bowman in Tuesday's primary -- which could mark the second major upset of a veteran New York City Democratic congressman in just two years.

However, the primary in New York’s 16th Congressional District was too early to call. As of 1 a.m. ET Wednesday, Bowman had roughly 61 percent of the counted vote. Engel was in second with about 36 percent of the counted vote.

New York counties didn’t release any mail ballots Tuesday, which could account for more than half the vote. Counties have until July 1 to start releasing the results of mail ballots.

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Engel, 73, serves as chairman of the House Foreign Affairs Committee. The panel's investigation into the firing of former State Department Inspector General Steve Linick drew a strong rebuke earlier this month from Secretary of State Mike Pompeo.

Engel's district encompasses southern Westchester County and the Bronx in New York City. He had secured a slew of powerful endorsers, including the Congressional Black Caucus, Majority Whip James Clyburn of South Carolina, House Intelligence Committee Chairman Adam Schiff of California, fellow New York Rep. Hakeem Jeffries and Massachusets Sen. Elizabeth Warren.

Hillary Clinton, New York Gov. Andrew Cuomo, and Senate Majority Leader Chuck Schumer had also thrown their support behind Engel.

But Bowman had the influential support of freshman U.S. Rep Alexandria Ocasio-Cortez, whose 2018 upset win over Democratic Caucus Chair Joe Crowley shocked the Democratic establishment.

Jamaal Bowman, who is running against Rep. Eliot Engel, D-N.Y., speaks during his primary-night party Tuesday, June. 23, 2020, in New York. (Associated Press)

"This moment requires renewed and revitalized leadership across the country AND at the ballot box," Ocasio-Cortez said in a tweet earlier this month. "Tonight, I am endorsing @JamaalBowmanNY for Congress."

For her part, Ocasio-Cortez on Tuesday defeated her challengers in the Democratic primary for New York’s 14th Congressional District.

Her chief challenger in Tuesday’s primary was Michelle Caruso-Cabrera, a former CNBC anchor who was a registered Republican until a few years ago.

The 30-year-old Ocasio-Cortez has been one of the most well-known voices of the American political left since her 2018 victory.

Her district in Queens and the Bronx was among the places hit hardest by the coronavirus, which made traditional campaigning impossible over the past three months.

House Foreign Affairs Committee Chairman Eliot Engel, D-N.Y., talks to reporters at the Capitol in Washington, Dec. 17, 2019. (Associated Press)

Other primaries

Meanwhile, voters rebuffed President Trump and nominated two Republicans he opposed to House seats from North Carolina and Kentucky on Tuesday. Calls in higher-profile races in Kentucky and probably New York faced days of delay as swamped officials count mountains of mail-in ballots.

In western North Carolina, GOP voters picked 24-year-old investor Madison Cawthorn, who uses a wheelchair following an accident, over Trump-backed real estate agent Lynda Bennett.

The runoff was for the seat vacated by GOP Rep. Mark Meadows, who resigned to become Trump's chief of staff and joined his new boss in backing Bennett.

U.S. Rep. Thomas Massie, R-Ky., a libertarian-minded maverick who often clashes with GOP leaders, was renominated for a sixth House term. Trump savaged Massie in March as a “disaster for America” who should be ejected from the party after he forced lawmakers to return to Washington during a pandemic to vote on a huge economic relief package.

Amy McGrath speaks to supporters in Richmond, Ky., Nov. 6, 2018. (Associated Press)

Cawthorn, who will meet the constitutionally mandated minimum age of 25 when the next Congress convenes, has said he's a Trump supporter, and Massie is strongly conservative. Still, their victories were an embarrassment to a president whose own reelection campaign has teetered recently.

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The Democratic Senate primary in Kentucky, where the winner will take on Senate Majority Leader Mitch McConnell, is too early to call. As of 10 p.m. ET on Tuesday, Amy McGrath had roughly 45 percent of the counted vote.

Charles Booker was in second with about 36 percent of the counted vote.

The Senate race is among the dozens of races in the state in which The Associated Press did not declare winners on Tuesday. They include all primary elections for state legislature, the Democratic primary elections for U.S. House in the state’s 4th and 6th Congressional Districts, and the Republican primary in the state’s 3rd Congressional District.

Fox News' Tyler Olson and The Associated Press contributed to this report.