Representative Jeff Van Drew of New Jersey, a moderate Democrat who is among his party’s staunchest opponents of impeaching President Trump, told aides on Saturday that he is planning to switch parties and declare himself a Republican as soon as next week, just as the House is casting its historic votes on articles of impeachment.

At a White House meeting on Friday, Mr. Van Drew sought Mr. Trump’s blessing for the move, which could be critical to his ability to avoid a primary challenge next year, and the president urged him to make the jump, according to two Democrats and one Republican who spoke on the condition of anonymity because they were not authorized to disclose the talks.

Mr. Van Drew has spoken with senior advisers to Mr. Trump about announcing his switch at an event at the White House either immediately before or just after the House votes on two articles of impeachment, which is expected to happen on Wednesday, according to Republicans and Democrats.