U.S. Rep. Jeff Van Drew of New Jersey was grappling with a wave of staff resignations on Monday following the weekend revelation that the freshman Democrat plans to defect to the Republican Party over his opposition to impeaching President Donald Trump.

Five senior aides who penned a joint resignation letter to Van Drew were set to hand in their badges at the Cannon House Office Building by the Capitol. At least six senior aides have resigned so far. More resignations from senior staff are expected within the week, according to one of the people who quit.

Van Drew’s imminent party switch, reportedly engineered with Trump’s support, promises to make the South Jersey House race one of the most competitive and closely-watched in the country. But Van Drew, who has only voted with Trump 7% of the time in Congress, will face an uphill battle in a Republican primary, where three candidates are already running.

The staffers who wrote the resignation letter said Van Drew’s “decision to join the ranks of the Republican Party led by Donald Trump does not align with the values we brought to this job when we joined his office.”

» READ MORE: New Jersey Republicans aren’t rushing to get out of Jeff Van Drew’s way

“We greatly respect Congressman Van Drew and are deeply saddened and disappointed by his decision,” they wrote to Allison Murphy, Van Drew’s chief of staff. “As such, we can no longer in good conscience continue our service in the congressman’s employ.”

The letter was signed by legislative director Javier Gamboa, communications director Mackenzie Lucas, deputy chiefs of staff Edward Kaczmarski and Justin O’Leary, and legislative assistant Caroline Wood. CeCe Doherty, director of constituency relations, also resigned on Sunday.

“Defeating Trump has and always will be the main goal for me,” Doherty said. “It’s the reason I got involved in politics. I could not, in good conscience, continue working in an office where mutual morals and values were no longer present.”

Van Drew didn’t immediately return a call seeking comment on Sunday.

He has repeatedly said he opposes the Trump impeachment inquiry because it has “torn our country apart," and that in voters in 2020, not Congress, should decide Trump’s fate. He was one of just two House Democrats who voted against the impeachment inquiry into Trump’s dealings with Ukraine.

Trump praised Van Drew’s coming party switch shortly after midnight on Sunday. “Wow, that would be big," Trump said on Twitter. "Always heard Jeff is very smart!”

Van Drew’s stance on impeachment eroded his support among Democrats in his district, and in recent days he received pressure from local party leaders urging him to vote yes on the articles of impeachment drafted by House Democrats.

"Jeff stabbed us in the back, certainly,” Michael Suleiman, the Atlantic County Democratic chairman, said Saturday. "It’s disgusting. It’s a disgrace. Good riddance.”

Murphy is expected to remain in her role as chief of staff. She stepped down from her post as Cape May Democratic state committeewoman Sunday morning. It was unclear whether she would register as a Republican.

New Jersey Democrats have reacted to Van Drew’s coming defection with outrage.

» READ MORE: Jeff Van Drew finally admits he was a Democrat in name only | Opinion

“Jeff Van Drew has chosen his political career over our Constitution," Gov. Phil Murphy said in a statement. “Despite knowing full well that the President has abused the powers of his office, Congressman Van Drew is now willing to enable Donald Trump just to try to salvage his own election.”

Van Drew’s district — which covers Atlantic, Cape May, Cumberland, and Salem Counties — leans conservative but is very much a swing district. Slightly more than 50% of the district voted for Trump in 2016, a five-point margin over Hillary Clinton. Barack Obama won it twice with 53% of the vote.

Van Drew, who was a longtime state lawmaker before being elected to Congress in 2018, had a little more than $900,000 in campaign cash as of Sept. 30, according to federal filings.