William Hague has told the Conservative conference that it was the "most crucial of our times" as the party seeks to show it is ready to govern.
Mr Hague: We are hungry for victory
The shadow foreign secretary said they would remind people the party should never be underestimated and that they were the "real change Britain needs".
Opening the conference, he said that in the "new world... the old politics of Gordon Brown are no longer enough".
He said Mr Brown had caused the current crises in health, prisons and pensions.
Mr Hague said: "He calculates that he can pretend to be a new government. But he is the old government, and after 10 years of failure and disappointment he cannot be the change the country needs."
With speculation continuing about whether or not Mr Brown is going to call an autumn election, the Conservatives have entered their conference week seeking to end the "Brown bounce" which has seen Labour ahead in opinion polls since Mr Brown replaced Tony Blair as prime minister.
Mr Hague, who won standing ovations at the beginning and the end of his speech, called for unity in the party, warning critics against "public self indulgence".
'Tenacity and intelligence'
To loud applause he poured scorn on Mr Brown's recent praising of, and appearance with, Baroness Thatcher at 10 Downing Street.
"Margaret Thatcher would never have devastated the pension funds of this nation, nor kicked its small businesses in the teeth, " he said.
"So you may fawn now at the feet of our greatest prime minister - but you are no Margaret Thatcher."
Mr Brown was not a "conviction" politician but was a "calculation politician".
By contrast, Tory leader David Cameron's "tenacity and intelligence" would help make a "remarkable and brilliant prime minister".
Mr Hague, who was Conservative leader from 1997 to 2001 before returning to the front bench under Mr Cameron, said Mr Brown had denied in 1997 that his new taxes would hit pension funds.
'Hungry for victory'
Mr Hague said recent figures put the cost to UK pension funds over the past decade at £100bn, which "makes Robert Maxwell look like an amateur".
"To me it is as simple as this: I do not believe that a politician who does such violence to the lifetime savings of millions of hardworking British people can be the right prime minister for our future," he said.
Mr Hague said: "The willingness of our country to vote for change depends on our ability to show that we are ready to fight back, to win and to govern."
"Let it ring out from our conference today: the Conservative Party is ready, it is hungry for victory and if Gordon Brown ever summons up the courage to call an election we are going to beat him," said Mr Hague.
At the Labour conference last week Mr Brown said he was a "conviction politician" who wanted to "defend and extend" British values and help everyone to reach their potential.
At the Lib Dem conference a week earlier, leader Sir Menzies Campbell said he aimed to "rattle the cage" of British politics and smash the "cosy consensus" between Labour under Mr Brown and the Conservatives under Mr Cameron.