The Conservatives say they are "firing the starting gun" for May's general election after launching their first campaign poster.
Prime Minister David Cameron said the alternatives offered by the other parties would be "disastrous".
But shadow chancellor Ed Balls accused the Tories of having an "extreme and ideological" approach to spending cuts.
And former Tory chairman Lord Tebbit said the party would struggle to win a majority unless UKIP "implodes".
With just over four months to go before polling day, BBC political correspondent Chris Mason said there would be plenty more campaign posters and newspaper articles in the weeks to come, with many focusing on the "central battleground" of the economy.
The new Conservative poster - with the slogan Let's Stay on the Road to a Stronger Economy - shows a road stretching out through countryside, and will appear on billboards across the country.
Launching it in Halifax, West Yorkshire, Mr Cameron said it would be "the most important election for a generation".
He said: "What is absolutely crucial is to win this forthcoming election because it is so important that we stay on the road to a stronger economy."
During a visit to Lib Dem Business Secretary Vince Cable's Twickenham constituency, Chancellor George Osborne said he was "firing the starting gun" of the general election race and said Labour's economic plans would lead to "chaos".
But shadow chief secretary to the Treasury Chris Leslie said Mr Osborne had promised there would be no deficit at all by 2015.
He said the chancellor had "neglected" what he called the "bread and butter question" of low and middle earners' wages, leading to a shortfall in tax receipts.
Earlier, writing in the Guardian, Mr Balls said the Conservatives had "lurched to the right" and ceded the "centre ground" of politics to Labour.
"Osborne's increasingly extreme and ideological approach goes far beyond the necessary task of deficit reduction," he wrote.
Lord Tebbit said Mr Cameron's failure to hit his target to reduce net migration to the tens of thousands was a "real problem" going into the general election.
He told BBC Radio 4's The World at One he expected former Tory MPs Douglas Carswell and Mark Reckless - who won by-elections last year after defecting to UKIP - to keep their seats.
He said: "The likely outcome of the general election looking at the polls over some months now, the Conservatives will not regain their votes to the level which they should do.
"I think it is going to be very difficult to win an overall majority unless, for some reason or another, UKIP should implode."
But current Tory chairman Grant Shapps said Margaret Thatcher, in whose government Lord Tebbit served, would "never have been fooled" by a party offering "easy solutions" on immigration.
He pointed to proposals to curb benefits for European migrants and the Conservatives' promise of a referendum on EU membership, adding: "There is no other route to get to that."
Commenting on the Conservatives' poster, a Lib Dem spokesman said: "The Conservative economic plan for the future looks more like the highway to hell for the majority of British voters, as the Tories want to roll back the state to the 1930s.
"Voters know that only the Lib Dems in a stable government can deliver the combination of economic responsibility and social justice that Britain needs."