WASHINGTON — Lawyers for Michael D. Cohen, President Trump’s former personal lawyer and fixer, told congressional lawmakers in a letter on Thursday that Mr. Cohen can best help their oversight inquiries if he remains out of prison to sift through millions of his documents — and they asked the help of Democrats in persuading prosecutors to help him do so.

The lawyers told Democratic members of congressional committees who have either subpoenaed his testimony or have asked him to appear voluntarily over the course of the past year that Mr. Cohen has been essential to their investigations, and they reminded them of the help they have sought from him even after his public hearing in February.

The lawyers — Lanny J. Davis, Michael D. Monico and Carly Chocron — wrote that Mr. Cohen was recently given access again to his files, which had been seized by federal investigators during a search in April 2018. The files include voice mail messages and recordings, as well as 14 million other documents, they said.

With Mr. Cohen expected to begin serving a three-year prison sentence on May 6, the lawyers asked the committee members to provide documentation of his help, in the hopes of showing that he is still needed to assist them with their investigations — and of postponing his incarceration or even reducing the time he will spend behind bars.