Today, a group of Democratic senators wrote to officials at the Federal Communications Commission and the Department of Justice, asking that they strike down a proposed merger deal between T-Mobile and Sprint. Sen. Richard Blumenthal (D-CT) led eight of his colleagues in the letters, which are addressed to FCC Chairman Ajit Pai and DOJ antitrust chief Makan Delrahim.

“Our enforcement officials are the last line of defense preventing reconsolidation of our telecommunications markets at the expense of American consumers,” the senators wrote. “We urge you to act to prevent this dangerous merger from proceeding.”

The group includes some of the Democratic Party’s most prominent leaders, such as Sens. Amy Klobuchar (D-MN), Kirsten Gillibrand (D-NY), Elizabeth Warren (D-MA), and Cory Booker (D-NJ), who have all announced that they were running for president in the past few weeks. Sens. Sherrod Brown (D-OH) and Bernie Sanders (I-VT), who are anticipated to launch 2020 campaigns soon, also signed on.

“We are deeply concerned that the merger of Sprint and T-Mobile in particular will eliminate competition that has been shown to benefit consumers and stifle the emergence of new carriers,” the senators added.

The proposed $26 billion merger between T-Mobile and Sprint was announced last April and is currently under review by both the FCC and the DOJ. If approved, the deal would shrink the United States’ major wireless carrier pool to three down from four, including AT&T and Verizon.

Executives from both T-Mobile and Sprint have said that the merger would help them deploy 5G wireless networks at a faster rate and would, in time, drive down consumer plan prices. In their letter, the senators argued that this just wasn’t the case.

“The best way to achieve the goal of high-quality, affordable, nationwide 5G is through competitive markets,” the senators wrote. “This merger moves us further away from the sort of competition we need to accomplish this aim. It will lead to excessive consolidation and undermine innovation.”

The letter comes amid an already tough week on Capitol Hill for the carriers. Lawmakers on the House Energy & Commerce Committee will hold a hearing n Wednesday to analyze the proposed merger deal. T-Mobile and Sprint executives are expected to testify at both hearings.

Updated 1/12/18 5:53 p.m. ET: This story originally said that the House Judiciary Committee would be holding a hearing on the T-Mobile-Sprint merger on Thursday. This hearing has been postponed.