income tax on Social Security benefits earned in USA.

I found the exact text regarding Double Taxation between the US and Portugal at the IRS website.  See Article 25 titled "Relief from Double Taxation".

2. In the case of an individual who is a citizen of the United States and a resident of Portugal, income that may be taxed by the United States solely by reason of citizenship shall be deemed to arise in Portugal to the extent necessary to avoid double taxation, provided that the tax paid to the United States will not be less than the tax that would be paid under the Articles of this Convention if the individual were not a citizen of the United States.

3. In the case of Portugal:
(a) Where a resident of Portugal derives income that, in accordance with the provisions of this Convention may be taxed in the United States (other than solely by reason of citizenship), Portugal shall allow as a deduction from the tax on the income of that resident an amount equal to the income tax paid in the United States. Such deduction shall not, however, exceed that part of the income tax, as computed before the deduction is given, that is attributable to the income that may be taxed in the United States;