The party was founded under the name Union for the Republic - Res Publica (Ühendus vabariigi eest - Res Publica), but was commonly referred to as simply Res Publica and changed its name to Res Publica Party.
The party won its first parliamentary elections (in 2003) after being established. Their election slogans were "Vote for new politics" ("Vali uus poliitika") and "Choose order" ("Vali kord"). They formed a coalition with the Reform Party and the People's Union. The coalition broke in 2005 as a result of opposition to Res Publica interior minister Vaher's policies and Res Publica has been in opposition since.
At the time of its merging with Pro Patria, it was led by Taavi Veskimägi, a former Minister of Finance. The previous chairman Juhan Parts was also Prime Minister from 10 April 2003 until 23 March 2005. Until the 2007 parliamentary elections, Res Publica was the largest party in the Riigikogu, with 28 of 101 seats, currently holding opposition.
On 4 April 2006, representatives of the Pro Patria Union and the representatives of Res Publica decided to merge the two parties. A decision was made to form a new party, named Union of Pro Patria and Res Publica (Isamaa ja Res Publica Liit), after approval by general assemblies of both merging political forces. Approval was given by the general assemblies on 4 June 2006. The union took third place in the 2007 elections, which, though they resulted in a loss in the combined parties’ strength in Parliament, was a stronger than expected showing.