Despite the fact the he had been in office for nearly 2 years, by the time of the 1950 midterms, President Thomas Dewey had no major legislative accomplishments to showcase to the American electorate as a reason to endorse his party. This, combined with Dewey's poor approval rating (34%), his controversial handling of the Korean War, and a nation tired of Joseph McCarthy's claims of communists having infiltrated the Democratic Party, meant Dewey's Presidency, much like Truman's 4 years earlier, felt "tired". And when a Presidency is tired, it's party suffers loses downballot. Dewey's Presidency would be no exception.
The Democrats picked up 5 seats in the Senate, pushing them above the 3/5ths mark. They defeated Republican incumbents in Missouri, Colorado, Iowa, Wisconsin, Indiana, and Idaho, (and came within a few points in Ohio) but lost the election in Idaho's other seat.
In the elections in the House of Representatives, the Democrats won their largest majority since FDR's 1936 landslide, and took a veto-proof majority in the chamber.