The Republicans held onto a seat in one of their strongholds, California, after a special election to replace a jailed Republican congressman.
Brian Bilbray will stand again in November's mid-terms
Brian Bilbray narrowly defeated his Democratic rival Francine Busby to take the House of Representatives seat.
The election has been closely watched as an indication of voter sentiment ahead of the key mid-terms in November.
The Democrats are hoping to win control of the House of Representatives from the Republicans.
The special election was called after congressman Randy "Duke" Cunningham resigned last year after admitting taking bribes.
He was jailed for eight years in March.
Mr Bilbray will serve out the remainder of Cunningham's term and is set to face Ms Busby again in November's mid-terms.
The Republicans and Democrats spent millions of dollars to campaign for the California election, which was seen as a key test of which party will win control of the House of Representatives.
Mr Bilbray put immigration at the centre of his campaign, while Ms Busby highlighted perceived public dissatisfaction with the Bush administration.
Several states held primaries on Tuesday to select candidates for the November elections.
In California, Phil Angelides narrowly beat Steve Westly to be the Democratic Party candidate to face incumbent Arnold Schwarzenegger in the election for state governor.