Nixon had some conservatives policies and some liberal policies, but:
1.He was the last president before Clinton who put forward a healthcare plan, which is apparently virtually identical to the plan Obama passed.
http://www.salon.com/2013/10/29/nixon_proposed_todays_affordable_care_act_partner/2.He was the last president of either party to propose a guaranteed annual income.
You're right Nixon wouldn't be considered today as the last liberal president, he'd be considered the last socialist president (prior to Obama, of course.)