Westerwelle joined the FDP in 1980.
He was a founding member of the Junge Liberale (Young Liberals), which became the party's official youth organisation in 1983, and was its chairman from 1983 to 1988.
In a 1988 newspaper interview, he singled out the FDP's rejection of an amnesty for tax offenders and its diminished enthusiasm for nuclear power as fruits of the youth wing's labors.

He was a member of the executive board of the FDP from 1988, and in 1994, he was appointed secretary general of the party.

In 1996, Westerwelle was first elected a member of the Bundestag, filling in for Heinz Lanfermann, who had resigned from his seat after entering the Ministry of Justice.
In the 1998 national elections, he was re-elected to parliament.
As his parliamentary group's home affairs spokesman, he was instrumental in swinging the FDP behind a 1999 government bill to make German citizenship available to children born in Germany of non-German parents.

In 2001, Westerwelle succeeded Wolfgang Gerhardt as party chairman.
Gerhardt, however, remained chairman of the FDP's parliamentary group.
Westerwelle, the youngest party chairman at the time, emphasized economics and education, and espoused a strategy initiated by his deputy Jürgen Möllemann, who, as chairman of the North Rhine-Westphalia branch of the FDP, had led his party back into the state parliament, gaining 9.8% of the vote.
This strategy, transferred to the federal level, was dubbed Project 18, referring both to the envisioned percentage and the German age of majority.
Leading up to the 2002 elections, he positioned his party equidistantly from the major parties and refused to commit his party to a coalition with either the Christian Democrats or the Social Democrats.
He was also named the FDP's candidate for the office of chancellor.
Since the FDP had never claimed such a candidacy (and hasn't done since) and had no chance of attaining it against the two major parties, this move was widely seen as political marketing alongside other ploys, such as driving around in a campaign van dubbed the Guidomobile, wearing the figure 18 on the soles of his shoes or appearing in the Big Brother TV show.
Eventually, the federal elections yielded a slight increase of the FDP's vote from 6.2% to 7.4%.
Despite this setback, he was reelected as party chairman in 2003.

In the federal elections of 2005, Westerwelle was his party's front-runner.
When neither Chancellor Gerhard Schröder's Social Democrats and Greens nor a coalition of Christian and Free Democrats, favored by Angela Merkel and Westerwelle, managed to gain a majority of seats, Westerwelle rejected overtures by Chancellor Schröder to save his chancellorship by entering his coalition, preferring to become one of the leaders of the disparate opposition of the subsequently formed "Grand Coalition" of Christian and Social Democrats, with Merkel as chancellor.
Westerwelle became a vocal critic of the new government.
In 2006, according to an internal agreement, Westerwelle succeeded Wolfgang Gerhardt as chairman of the parliamentary group.

Over the following years, in an effort to broaden the party's appeal, Westerwelle embraced its left wing under former justice minister Sabine Leutheusser-Schnarrenberger and focused his campaign messages on tax cuts, education and civil rights.
Westerwelle joined the FDP in 1980. He was a founding member of the Junge Liberale (Young Liberals), which became the party's official youth organisation in 1983, and was its chairman from 1983 to 1988. In a 1988 newspaper interview, he singled out the FDP's rejection of an amnesty for tax offenders and its diminished enthusiasm for nuclear power as fruits of the youth wing's labors.
He was a member of the executive board of the FDP from 1988, and in 1994, he was appointed secretary general of the party.
In 1996, Westerwelle was first elected a member of the Bundestag, filling in for Heinz Lanfermann, who had resigned from his seat after entering the Ministry of Justice. In the 1998 national elections, he was re-elected to parliament. As his parliamentary group's home affairs spokesman, he was instrumental in swinging the FDP behind a 1999 government bill to make German citizenship available to children born in Germany of non-German parents.
In 2001, Westerwelle succeeded Wolfgang Gerhardt as party chairman. Gerhardt, however, remained chairman of the FDP's parliamentary group. Westerwelle, the youngest party chairman at the time, emphasized economics and education, and espoused a strategy initiated by his deputy Jürgen Möllemann, who, as chairman of the North Rhine-Westphalia branch of the FDP, had led his party back into the state parliament, gaining 9.8% of the vote. This strategy, transferred to the federal level, was dubbed Project 18, referring both to the envisioned percentage and the German age of majority. Leading up to the 2002 elections, he positioned his party equidistantly from the major parties and refused to commit his party to a coalition with either the Christian Democrats or the Social Democrats. He was also named the FDP's candidate for the office of chancellor. Since the FDP had never claimed such a candidacy (and hasn't done since) and had no chance of attaining it against the two major parties, this move was widely seen as political marketing alongside other ploys, such as driving around in a campaign van dubbed the Guidomobile, wearing the figure 18 on the soles of his shoes or appearing in the Big Brother TV show. Eventually, the federal elections yielded a slight increase of the FDP's vote from 6.2% to 7.4%. Despite this setback, he was reelected as party chairman in 2003.
In the federal elections of 2005, Westerwelle was his party's front-runner. When neither Chancellor Gerhard Schröder's Social Democrats and Greens nor a coalition of Christian and Free Democrats, favored by Angela Merkel and Westerwelle, managed to gain a majority of seats, Westerwelle rejected overtures by Chancellor Schröder to save his chancellorship by entering his coalition, preferring to become one of the leaders of the disparate opposition of the subsequently formed "Grand Coalition" of Christian and Social Democrats, with Merkel as chancellor. Westerwelle became a vocal critic of the new government. In 2006, according to an internal agreement, Westerwelle succeeded Wolfgang Gerhardt as chairman of the parliamentary group.
Over the following years, in an effort to broaden the party's appeal, Westerwelle embraced its left wing under former justice minister Sabine Leutheusser-Schnarrenberger and focused his campaign messages on tax cuts, education and civil rights.
Guido Westerwelle