Philipp Roesler's Free Democratic Party also sought to put recent infighting over his future behind it, giving the 40-year-old another two-year term as leader.
Poll ratings for the FDP, the junior partner in conservative Chancellor Angela Merkel's coalition government, are below the 5 percent needed to win parliamentary seats—a complicating factor as Merkel seeks a third term.
The FDP entered Merkel's government after winning nearly 15 percent of the vote in Germany's 2009 election with a campaign that focused on big tax cuts. However, it failed to achieve them and received much of the blame for frequent coalition squabbling.
At a party convention on Saturday, Roesler, who is also Germany's economy minister, asserted that the center-left opposition plans "an orgy of tax increases" and his party is the only bulwark against that. Merkel's party "has long since toppled over in the fight against extra burdens, but doesn't dare tell people that," he said.
Roesler's party has often talked particularly tough in the eurozone debt crisis, in which Merkel has pushed an austerity-heavy remedy that is unpopular abroad.
He praised efforts to cut debt and implement economic reforms, but said there is "a new danger—discussions over whether it would perhaps be better, instead of strengthening competitiveness, to weaken the currency." France recently urged eurozone nations to help control the euro's value.
Roesler said his party "keeps the coalition on course," rejecting calls for a mandatory national minimum wage and pressing Merkel's conservatives to overcome their reluctance to strengthen rights for gay couples.
Roesler, the FDP leader since May 2011, long appeared likely to lose his job as the party's poll ratings stagnated. However, he was strengthened by an unexpectedly strong performance in a January state election, which allowed him to wrong-foot critics.
On Saturday, he ran unopposed for re-election as leader and was backed by some 86 percent of delegates. Parliamentary group leader Rainer Bruederle, long considered a potential replacement, will lead the party's election campaign.
"If we stand together, if we stick to our guns, we will be successful," Roesler said.
Merkel's Union bloc currently leads the polls, but they give neither her current coalition nor center-left rivals a majority.