WASHINGTON — A former Iowa state senator pleaded guilty Wednesday to receiving and concealing payments in exchange for switching his support from one presidential candidate to another in the 2012 election, the Justice Department said.

The former lawmaker, Kent Sorenson, resigned from the Iowa Senate last year after an investigation found that he probably violated ethics rules by taking money from presidential campaigns.

Mr. Sorenson, 42, of Milo, Iowa, had been the state chairman for the presidential campaign of Representative Michele Bachmann, Republican of Minnesota, but then switched his support to former Representative Ron Paul of Texas just days before the state’s caucus.

In a statement filed with his plea agreement, Mr. Sorensen admitted that he agreed to switch his allegiance in exchange for $73,000 in payments. The Justice Department did not name the campaigns involved in the scheme. He has not yet been sentenced.