Two top Senate Democrats are opening their own investigation of the U.S. Postal Service, aiming to explore whether changes to mail processing and delivery have prevented seniors, veterans and other patients from receiving their prescription drugs on time.

The new inquiry is the work of Sens. Elizabeth Warren (D-Mass.) and Robert P. Casey Jr. (D-Pa.), who issued a battery of letters to CVS, Walgreens and other top providers on Thursday to determine whether their customers had experienced delays in recent weeks. They cited public reporting and private correspondence with their constituents as they raised concerns that patients may have been forced to go without medication for weeks as a result of policies enacted under the watch of the Trump administration.

“Millions of Americans with diabetes, high blood pressure and heart disease, asthma, and other chronic conditions, illnesses or health care needs rely on the USPS for delivery of their prescription drugs and are at grave risks if President Trump’s efforts to degrade mail service results in delays and disruptions,” they wrote. “These health threats are magnified by the ongoing pandemic.”

The two Democrats directed their letters to Cigna Corp., which owns Express Scripts; CVS Health; UnitedHealth Group, which owns OptumHealth; Humana; and the parent company for Walgreens and Duane Reade. They asked them to supply information about the number of customers receiving their prescriptions by mail, the average time it takes for them to be shipped and the extent to which recent Postal Service policy changes have introduced new delays for customers or costs to the companies.

The Postal Service declined a request for comment.

The lawmakers’ missive Thursday reflects the mounting frustration among Democrats with the USPS and its leader, Postmaster General Louis DeJoy, a fury likely to be laid bare when he and Robert M. Duncan, the leader of the Board of Governors that oversees the Postal Service, testify in front of lawmakers at hearings that will begin Friday.

DeJoy, who has led the agency since mid-June, enacted broad operational changes to return the Postal Service to solvency — it’s carrying $160.9 billion in debt, most of which is connected to retiree obligations — amid declines in mail volume. But his cost-cutting campaign ultimately reduced postal workers’ hours and overtime and in some cases resulted in the removal of mail-sorting machines at post offices nationwide, sparking a political outcry.

Democrats in particular see the potential for a catastrophic logjam that could undermine the 2020 presidential election, when millions of Americans are expected to mail their ballots — as opposed to casting them in person — as a result of the coronavirus pandemic. The withering public pressure ultimately forced DeJoy this week to announce that he would suspend some of his measures, but many congressional Democrats say they remain unconvinced.

On Thursday, Warren and Casey called DeJoy’s commitments “vague” and fretted that the potential disruption to mail-order prescriptions comes at a time when the pandemic is forcing patients to take advantage of such deliveries instead of visiting their doctor’s offices or pharmacies in person.

Some national pharmacy chains have signaled that they have not actually been affected by some of the Postal Service’s new policies. Representatives for RiteAid, Express Scripts, CVS and Walgreens each said they have not experienced unusual disruptions, The Post reported earlier this week.

But their statements still contrast with the stories that have flooded social media in recent days from seniors and veterans nationwide. Casey in particular said his office has collected more than 15,000 messages about mail-delivery concerns, and he and Warren added in their letters Thursday that they had received additional reports about the effect on prescription drugs.