Senator Bernie Sanders of Vermont is seeking to bar allies of Hillary Clinton from leading the powerful rules and platform committees of the Democratic National Convention in July, escalating his battle with party leaders.

In a letter sent on Friday to party officials, lawyers for Mr. Sanders said that the appointments of Barney Frank, the former Massachusetts congressman, and Gov. Dannel P. Malloy of Connecticut violated party rules. Mr. Frank is to co-lead the rules committee, and Mr. Malloy the platform committee.

In the letter, Mr. Sanders’s lawyer Brad Deutsch said that both men have been “harsh, vocal critics of Senator Sanders, and equally active supporters of his challenger, Hillary Clinton.” Mr. Frank has called Mr. Sanders “outrageously McCarthyite” for his suggesting that Mrs. Clinton would be influenced by her speaking fees from Wall Street; Mr. Malloy has led efforts among Clinton allies to attack Mr. Sanders’s record on gun control.

Under convention rules, Mr. Deutsch said in the letter, their open criticism of Mr. Sanders made them unfit to co-lead the committees.