In 1954, the Congressman in Iowa's 1st congressional district, Thomas E. Martin, ran for the U.S. Senate. Schwengel ran and won the Republican nomination for the seat, and easily defeated John J. O'Connor in the general election. He won re-election in the following four elections. However, in the 1964 Democratic landslide, Schwengel (like all but one of Iowa's Republican U.S. House members) was defeated. He lost to University of Iowa political science Professor John R. Schmidhauser by fewer than 4,000 votes. But Schmidhauser, like many members of the 1964 freshman class, served only one term; in 1966 Schwengel ran again for his former seat and defeated Schmidhauser by fewer than 5,000 votes, then defeated him again in 1968 by a wider margin. In all, Schwengel served eight terms in Congress.