In their first appearance on the national stage, the 1999 European Parliament election in Italy, the SDI won 2.2% of the votes and two MEPs.

For the 2001 Italian general election, the SDI formed an unusual alliance (The Sunflower) with the Federation of the Greens, which was disbanded soon after the election due to political divergences and ultimately to the disappointing electoral result: 2.2% of the vote, while the two parties' combined result in 1999 had been 4.0%.

In 2005, the SDI entered in alliance with the Italian Radicals, forming the Rose in the Fist (RnP) electoral list.

In the 2006 Italian general election, the RnP list won only 2.6% of the vote, much less than the combination of the two parties before the alliance, as the Radicals lost voters in their strongholds in the North to Forza Italia, while the Socialists lost ground in the South to The Olive Tree parties.

[6] In May, Del Turco and his faction left the SDI and formed the Reformist Alliance (Italian: Alleanza Riformista),[7] which would merge into the PD.