Nick Clegg will insist the Liberal Democrats are no longer a party of protest as he urges activists to attack the Tories.
The Deputy Prime Minister will hail the by-election victory in Eastleigh as proof that the Lib Dems can succeed as a party of government. He will also dismiss the "myth" that being in coalition has undermined their identity or ability to fight the Conservatives.
The rallying cry comes in a keynote speech to the Lib Dem spring conference in Brighton. The leadership has been eager to use the gathering to move on from damaging allegations of sexual harassment by Lord Rennard - which he denies - and the conviction of former Cabinet minister Chris Huhne.
Mr Clegg will round on critics who wrote the party's "political obituaries" as the controversies threatened to overshadow the by-election campaign last month.
"The odds were stacked against us. A fierce campaign, under a national spotlight, dogged by difficult headlines from day one. Extraordinary circumstances. Yet we still won," he is expected to say.
"We beat the Tories. We squeezed Labour - don't forget that bit. Why? Because, for the first time in a generation we could campaign on our record of local delivery and our record of national delivery too... We didn't win in Eastleigh in spite of being in power. We won in Eastleigh because we're in power - locally and nationally."
Mr Clegg will concede that some Lib Dems had harboured "quiet fears" that coalition would do irreparable damage to the party. But he is to insist that the risk can pay off at local elections in May and the next general election in 2015.
"There is a myth that governing together, in coalition, diminishes the ability of the smaller party to beat the bigger party. The idea that, in Tory-facing seats the Liberal Democrats will find it impossible to distinguish our record, our values, from theirs," Mr Clegg is to say.
"But that myth has been utterly confounded. The opposite is true. The longer you stand side-by-side with your opponents the easier your differences are to see. We don't lose our identity by governing with the Conservatives. The comparison helps the British people understand who we are."
Mr Clegg will continue: "In the days after the by-election, even though we won, I was asked how I feel about our party no longer being a magnet for the protest vote. No longer the automatic 'none-of-the-above' choice. But the truth is: the Liberal Democrats are not a party of protest, we are a party of change. A party that is for things, not simply against things."