WASHINGTON — Representative Justin Amash, the Michigan libertarian who left the Republican Party this month, withdrew from the House Oversight and Reform Committee on Monday, leaving an influential post where he had joined Democrats in their efforts to investigate President Trump.

The formal withdrawal from both the House Republican Conference and the committee, which Mr. Amash submitted to Republican leadership on Monday, leaves Democrats without any real Republican ally on one of the powerful committees pursuing investigations into the president.

Even before he became the only Republican to declare his support for an impeachment inquiry, Mr. Amash had begun joining Democrats on the committee in approving subpoenas for administration officials, giving them at least the nominal claim to bipartisanship.

He announced his decision to leave the Republican Party on Independence Day, in a Washington Post opinion piece that he attached to his letter of resignation as an explanation for his departure.