From a bit of quick research, it appears that the descamisados (shirtless ones) were the working class poor of Argentina and would have probably included many of the indigenous Argentinian indians. As such, it would have been the (ex-European) whites who were not part of the original inhabitants of Argentina.
Because of their numbers and his promotion of their cause, it would have been comparatively easy for Peron to recruit and use them as his power base. In return he could offer them the possibility of freedom from oppression and, as you suggest, reabsorption into the new nation.
Hope this helps
- For Teachers