Washington (CNN) Much of the staff serving a freshman Democratic congressman resigned on Sunday after sources said the lawmaker prepares to switch to the Republican Party, according to a signed letter obtained by CNN .

Five aides to New Jersey Rep. Jeff Van Drew -- Javier Gamboa, Edward Kaczmarski, Justin M. O'Leary, Mackenzie Lucas and Caroline Wood -- wrote in the letter that they can "no longer in good conscience continue our service in the Congressman's employ."

A sixth staffer also resigned Sunday evening, a source with knowledge of the matter told CNN.

Among the staffers who quit were Van Drew's legislative director, communications director and his scheduler.

"We greatly appreciate the opportunities that the Congressman has given us, and we are proud of the work we've done together on behalf of the people of New Jersey's Second Congressional District," the letter said. "Sadly, Congressman Van Drew's decision to join the ranks of the Republican Party led by (President) Donald Trump does not align with the values we brought to this job when we joined his office."

The resignations come as Van Drew, who strongly opposes House Democrats' impeachment of Trump, has told colleagues of his intentions to switch parties, according to two sources.

Internal polling has shown he was losing major support from Democrats in the district, Democratic sources told CNN on Saturday. Switching parties would allow Van Drew to avoid a Democratic primary challenge for his seat.

Van Drew was one of just two Democrats who in October voted against formalizing the impeachment inquiry , and announced earlier this month that he would vote against all articles of impeachment on the House floor. The full House of Representatives is expected to vote this week on two House Judiciary Committee-approved articles of impeachment , which will set up a trial in the Senate.

After voting against the formal inquiry in October, the New Jersey congressman said in a statement that "without bipartisan support I believe this inquiry will further divide the country tearing it apart at the seams and will ultimately fail in the Senate."

News of Van Drew's intention to switch parties drew swift condemnation from Democrats.

New Jersey Gov. Phil Murphy on Saturday assailed Van Drew, who represents a district where the Republican President is popular, as "willing to enable" Trump "just to try to salvage his own election."

"Betraying our values by siding with Donald Trump is the final straw and made it impossible for him to continue being supported by our party, as grassroots activists, local party leaders in his district, and I have made clear in recent weeks," Murphy, a Democrat, said in a statement. "This party switch is cynical and desperate, and I am confident that a Democrat who shares the values and priorities of our Democratic Party will hold this seat."

Democratic Rep. Steve Cohen of Tennessee told CNN's Ana Cabrera on "Newsroom" that Van Drew is "making a serious mistake."

"I understand that he feels if he votes against impeachment, he'll lose in a Democratic primary and he could," Cohen said Sunday. "But you know he got elected with Democratic votes under a Democratic banner which he ran under for 30 years or so. I think he was a Senator and a Mayor and a Representative and all, and he got Democratic money including Democratic Congressional Campaign Committee money, and to turn and go to be a Republican, it's kind of strange."

He added: "I've heard of rats jumping off a sinking ship, but very few of them jump onto a sinking ship."

Van Drew's district -- a district that had voted for Barack Obama in 2008 and 2012, before voting for Trump in 2016 -- had been held by former Republican Rep. Frank LoBiondo for 24 years before Van Drew won the seat in 2018. In that race, Van Drew ran against a Republican candidate, who after making controversial remarks against diversity, lost the support of the National Republican Congressional Committee.

On Saturday, House Minority Leader Kevin McCarthy, a California Republican, welcomed Van Drew to the Republican Party.

"I want to tell Jeff Van Drew that he is welcome in the Republican Party, not just by me but by our conference, and we would support him and we would welcome him to join," McCarthy told Fox News' Jeanine Pirro.

UPDATE: This story and headline have been updated to reflect that a sixth staffer resigned Sunday.