Sen. Elizabeth Warren. (Flickr Photo / Elizabeth Warren)

Elizabeth Warren continued her crusade against Big Tech on Monday. The Massachusetts senator once again raised concerns that Amazon is behaving anti-competitively in a tweet.

Driving the news: The Wall Street Journal reported that Amazon is testing a new feature in its app that promotes its own brands over third-party sellers more aggressively than previously thought. Amazon is testing a pop-up window suggesting a lower-priced, private label item that appears over product pages for rival products.

(Update) What Amazon is saying: A spokesperson for the Seattle tech giant provided the following statement.

We regularly experiment with new shopping experiences for customers and this was a small test. The similar, lower priced product options shown to customers featured relevant items from a range of brands on our website and were displayed when a customer clicked on any type of listing.

Why it matters: European — and increasingly U.S. — regulators are concerned that Amazon’s move into private label brands is anticompetitive because the company operates a marketplace and competes on it.

Big picture: Warren has pledged to break up Big Tech in her bid for the White House in 2020. She’s not alone. A growing cohort of progressive legislators, including Senators Bernie Sanders and Amy Klobuchar, are vowing to crack down on the power that big tech companies wield.

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