WASHINGTON — Lawyers for Paul Manafort, President Trump’s former campaign chairman, fought back Monday against the government’s portrait of their client as a hardened criminal, suggesting that he was facing the prospect of dying in prison only because he had been unfairly caught in the scandal over Russia’s interference in the 2016 presidential race.

In a sentencing memorandum to the federal judge who will decide his punishment next month for two felonies to which Mr. Manafort pleaded guilty last year, his legal team said his crimes did not warrant a substantial prison term, especially given his age. Mr. Manafort will turn 70 in early April.

Disputing the prosecutors’ damning characterization of Mr. Manafort in a court filing last week, his lawyers insisted that Mr. Manafort was not only deeply remorseful, but “has suffered almost unprecedented public shame” for what they called garden-variety offenses. The memos are previews of the oral arguments that both sides will make before Judge Amy Berman Jackson of the United States District Court for the District of Columbia, who will sentence Mr. Manafort for the two conspiracy charges on March 13.