US President Donald Trump has announced that Jeff Van Drew, a New Jersey Democrat who broke with his party to vote against Mr Trump's impeachment, is officially switching parties and becoming a Republican.

Key points: Mr Van Drew's district in New Jersey has become increasingly conservative

Mr Van Drew's district in New Jersey has become increasingly conservative He said the move to the Republican Party was a "better fit" for him

He said the move to the Republican Party was a "better fit" for him Mr Trump has also endorsed Mr Van Drew's campaign for re-election

"Jeff will be joining the Republican Party … It's a big deal," Mr Trump said during an Oval Office event with Mr Van Drew, who broke the news to his staff over the weekend, prompting widespread resignations.

Mr Van Drew broke with his party and voted against impeaching Mr Trump — a move that bolstered the Republican Party's attempts to depict Democrats as divided on the matter.

Republicans voted unanimously against impeaching the President.

"I believe that this is just a better fit for me," Mr Van Drew said of his decision.

He told Mr Trump the final straw had been a meeting with a local Democratic Party county chairman who threatened to "destroy" him if he voted with Republicans on impeachment.

"This is who I am, it's who I always was, but there was more tolerance of moderate Democrats, of Blue Dog Democrats, of conservative Democrats," Mr Van Drew said, adding that he believed that tolerance was "going away".

Mr Trump, revelling in the decision, offered his support and announced that he was endorsing Mr Van Drew for re-election, calling him "a tremendous asset for the party".

Mr Van Drew later told reporters that having the President support his campaign was "helpful".

Mr Van Drew has previously differed with fellow Democrats on issues like gun control and gay marriage. ( AP: Evan Vucci )

Democratic House Speaker Nancy Pelosi said she had not discussed the move with Mr Van Drew.

Asked by reporters whether she had advice for him, she responded: "Nothing. Zero."

Mr Van Drew met last week with Mr Trump, who praised the congressman on Twitter on Tuesday.

"Congressman Jeff Van Drew is very popular in our great and very united Republican Party," the President wrote.

"It was a tribute to him that he was able to win his heavily Republican district as a Democrat. People like that are not easily replaceable!"

Mr Van Drew, 66, had been a conservative state senator before he joined Congress, bucking Democrats on issues including gun control and gay marriage.

His district has become increasingly conservative, with Mr Trump carrying it narrowly in 2016 after Barack Obama won it in 2008 and 2012.

AP