A new 50p top rate of income tax for high earners should only be a temporary measure, Chuka Umunna said.
Labour is committed to reintroducing the higher charge on those earning over £150,000 that was ditched by the coalition government.
But the shadow business secretary said how long to sustain the measure should be a purely economic decision and not a moral one.
"I wouldn't want to do it permanently because ... I would like to see the tax burden as low as possible," he told the New Statesman magazine.
"I don't believe that you tax for the sake of taxing: you tax to fund public services and, currently, to reduce our deficit and our debt."
Why we should merge income tax and NI
During the Labour leadership race in 2010, Ed Miliband said the 50p rate should stay in place and was "not just about reducing the deficit, it's about fairness in our society".
Shadow chancellor Ed Balls has insisted it is "not a matter of principle"
Mr Umunna hit out at the Green Party and others on the left of the political spectrum who "simply seek to set their face against the world as it is.
"Actually, the real challenge for responsible parties that are looking to govern is: 'How do you embrace these forces and make them work for the most people possible?".
And he said the election campaign was giving Mr Miliband the chance to show the public that he had been underestimated by critics.
"I always thought the more people saw of Ed, the more they would like him.
"One of the things that the Tories have massively underestimated in this campaign is the extent to which the characteristics that people attribute to Ed - honesty, decency, integrity - are an asset to our party."