When calculating 2019 1040 ES payments, to avoid penalty for underpayment, what does "taxes paid last year" mean? Fed Income Tax only, or SE tax and Medicare too?

One way to avoid penalty is pay 100% of taxes paid last year.  Does "taxes paid last year" mean just income tax, or does it include the employee portion of SS and Medicare paid last year?  A related question is, if you start a tax year with taxes withheld, then change to 1099 income, will you still get hit with a penalty?