WASHINGTON — Stephen K. Bannon, the former White House chief strategist, is forming a group to try to sell voters on a midterm message that they should support Republicans to defend the Trump agenda and save the president from impeachment.

The group, Citizens of the American Republic, is Mr. Bannon’s new vehicle after he was pushed out of the White House last summer and then parted ways with the conservative website Breitbart.

Mr. Bannon declined to describe his donors or how much money the group has raised. One person close to the group said that Rebekah Mercer, the hedge fund billionaire who supported Mr. Bannon’s previous political efforts but broke ties with him months ago, has no role.

Most of Mr. Trump’s advisers in the White House consider Mr. Bannon’s foray into the midterms to be quixotic — they say the House is almost certainly lost to the Democrats — and see his efforts as another way to promote himself. They point to Mr. Bannon’s paltry record in backing Republican candidates, particularly Roy S. Moore, the Alabama Senate hopeful who was accused of sexual misconduct against several women, including a teenager, and lost last year to a Democrat.