To have an argument against power — that is the central concern of politics. Without that, there is only power against power.
Wealth is just an extension of power. It is not a separate justification, contrary to the pretenses of libertarians. Yet in this world globalized by empires and capital, how do you have an argument against force and finance?
Perhaps by exposing the political power of wealth; few people think corporations should have more power than voters. And perhaps by exposing the violence underlying the accumulation of wealth, and the destructiveness of this accumulation in the context of climate change.
But a critique presupposes a standard against which to critique, some positive value imbuing a vision of a different way. The value underlying the critique of power is, ultimately, equality. The expression of equality may be thought of as harmony instead of exploitation, not only among people who have been divided into classes based on socioeconomic status and cultural identity, but most fundamentally harmony of humans and Earth.
A political party could organize campaigns around such a vision: a green economy, in which public goods are valued as such, in which preservation and conservation are measures of monetary value, and depletion and pollution are internalized as costs. The rhetoric must make clear that such an economy is not just the dream of radical, mostly white, environmentalists; its greatest beneficiaries would be the working class of every race. The only class who stands to lose are corporations and the few individuals who have benefitted from the concentration of corporate wealth in an economy based on maximal private appropriation of public resources.