Democrats are becoming more liberal, and it isn't hurting them at all. This is objectively true, and I have two charts to prove it.
First, as shown in the chart at the top of this post, long-term polling trends from Gallup clearly show that an increasing percentage of Americans who self-identify as Democrats also self-identify as liberals.
In 2002—the year Daily Kos was founded—there were almost as many conservative Democrats as liberal Democrats, and moderate Democrats far outnumbered either liberals or conservatives. Now, there are actually more liberal Democrats than moderate Democrats, and conservative Democrats have become a small minority.
What's more, as Democrats have become more liberal, they have not lost any ground to Republicans, as this next chart from Gallup shows:
Over the past decade as Democrats have become more liberal, if anything they have improved their position relative to Republicans. They certainly have not lost ground.
The polls used in this chart have enormous numbers of participants, making even small, one percent changes statistically significant.
So there you have it. Objective proof that becoming more liberal doesn't hurt Democrats, not even a little bit.