That shows a remarkable lack of understanding about how democracy works. Voters decide who makes up those parties. And Democrats especially are not all the same. Many of them are not imperialists. Unfortunately, many of them are...but it isn't universal. And the only way to tip the balance against imperialism, is to run leftists for every possible position there is. Up and down the ballot.
But, then you have to actually show up and vote for them when they are in the race. If you don't...then nothing will ever change.