For the majority of Americans, all of their income is subject to taxation for social security. However, any income accumulated beyond $127,200 (as of 2017) is exempt from the tax. This cap makes the social security tax regressive where, effectively, the burden of funding social security falls on the middle and working classes while the wealthiest Americans pay less than their fair share.
I support removing the cap. Doing so will ensure the solvency of social security for at least 50 more years and permit the expansion of benefits. Furthermore, there is no justifiable reason for not subjecting income earned over an arbitrary threshold to the social security tax.