Liberal Judaism is dying from liberalism. Orthodox Jews have kids and lots of them.
There is no such thing, at least from a religious branch, as “Liberal Judaism.” In the U.S. the 3 most common branches are Reform, Conservative, and Orthodox. Within Orthodox there are micro branches such as Modern Orthodox & Lubavitch.
From a political perspective Conservative Jews are the most liberal, probably 90/10 split. Reform is slightly more mixed at 60/30 for example, Bernard Marcus, co-founder of Home Depot, is Reform, and also very Republican, esp. on economic issues. Reform Judaism is the least observant to traditional Jewish tenants. For example, they do not believe Kosher is a relevant concept and eating pork, shellfish, ect., is not regarded as a major sin.
However, Conservative Judaism is the largest branch in the U.S. Ironically, most are neither all that religious or politically conservative but it was the branch of their ancestors so it’s part of their identity.