A top Senate Democrat is taking aim at Comcast over net neutrality, arguing that its previous commitments to an open Internet won't be enough to protect consumers — particularly if its merger with Time Warner Cable goes through.

In a letter to the company Monday, Senate Judiciary Committee Chairman Patrick Leahy (D-Vt.) said Comcast should pledge to avoid "paid prioritization" schemes that would speed up some Web site traffic over others.

Though Comcast has said it has no plans to implement paid prioritizaion, Leahy wrote, "I welcome that assertion, but I remain gravely concerned that if such agreements are permitted, market incentives may drive Comcast and other Internet service providers (ISPs) to change that position in the future."

Comcast said that it was reviewing the letter and declined to comment further. The company is already bound by the Federal Communications Commission's old net neutrality rules, thanks to a condition imposed on its 2011 merger with NBC Universal, and it has volunteered to extend those protections to any new Time Warner Cable (TWC) subscribers it picks up in this latest merger.

But consumer advocates worry about the longevity of that commitment, which expires in 2018. Beyond that date, Comcast will not be required to observe any rules other than what the FCC approves as a successor to the old net neutrality regulations, which themselves were struck down in January by a federal court.

Leahy's letter won't be a surprise to folks who've been following the Comcast merger or net neutrality; the senator has held two hearings on the open Internet this year. He's also preparing legislation with House lawmakers that would ban paid prioritization.

Still, tying the Comcast-TWC merger to net neutrality is a signal that Leahy might like to see a ban on paid prioritization imposed as a condition for the deal. The FCC and the Justice Department are expected to decide the fate of the proposed merger later this year.