Interestingly, a multi-party political system, while preferable is not a prerequisite for an extant democracy. For example the current Republican Party while professing to be "for the people"1 excludes many of our citizens from political participation, usually under the rubric of "States Rights" using a variety of gimmicks, ruses and artifices. If you can exclude citizens then any responsiveness to their discontent and interests becomes irrelevant. Assuming the Democrats' leaders continue providing a balanced response to the discontent and interests of all citizens you've created a de facto single party democracy.
The Republican Party and its followers' use of dogma to obviate their responsiveness to our citizenry's discontent and interests strengthens the case for a de facto single party democracy. According to their dogma "God, Markets, and Prayers" are the responsive constructs to our citizenry.
The Democratic Party must stop pretending that the Republican Party is part of our multi-party system of democracy and call it what it is - a dogma spewing deceptive dictatorship requiring immediate removal and replacement.
1. The Republican Party is highly likely to be "for the people" - just not for the people participating in their own governance.