(Corrects paragraph 4 to show DeJoy is a supporter of Trump whowas named head of the Postal Service last year, not named byTrump to head Postal Service)

By David Shepardson

WASHINGTON, Feb 17 (Reuters) - U.S. mail deliveries arestill facing "unacceptable delays" months after the problemfirst appeared and Postmaster General Louis DeJoy should explainwhy he has not fixed the issue, according to a group of 33 U.S.senators on Wednesday.

During the week ended Dec. 26, nationwide on-time deliverywas 64% for first-class mail and 45% for periodicals, thesenators said.

Delays in paychecks and other mail deliveries by the U.S.Postal Service (USPS) gained attention this summer as a recordnumber of voters mailed in ballots to elect a new president.

DeJoy, a supporter of former President Donald Trump who wasnamed head of the Postal Service last year, suspendedoperational changes in August after enormous criticism overpostal delays.

He is set to testify next week before a U.S. House ofRepresentatives committee on the Postal Service's financialoutlook along with will Ron Bloom, a former Obama administrationofficial elected last week as the new chairman of the U.S.Postal Board of Governors.

The 31 Democratic senators and two independent lawmakers,led by Michigan Democrat Gary Peters who chairs the committeeoverseeing the Postal Service, said "we urge you to be fullytransparent with the public about Postal Service operations andthe reasons they are still facing delays."

They cited reports USPS as part of a forthcoming 10-yearstrategic plan may further slow mail.

DeJoy said Wednesday in a statement the plan aims to fixproblems "that are preventing the Postal Service from meetingthe American people’s expectations for reliability, andresulting in billions of dollars of losses every year with noend in sight."

DeJoy came under heavy criticism for making service changesthat delayed deliveries and he suspended them in August ahead ofthe 2020 presidential election.

"We must acknowledge that during this peak season, we fellfar short of meeting our service targets. Too many Americanswere left waiting weeks for important deliveries of mail andpackages," DeJoy said last week, apologizing to customers.

Last week, USPS reported $318 million of income for thequarter ended Dec. 31, delivering a record 1.1 billion holidayseason packages, while first-class mail revenue decreased by$177 million.

USPS reported net losses totaling $86.7 billion from 2007through 2020. One reason is Congress in 2006 passed legislationrequiring USPS to pre-fund more than $120 billion in retireehealth care and pension liabilities. Labor unions have calledthis requirement an unfair burden that other businesses do notshare.

(Reporting by David Shepardson; Editing by Leslie Adler andLisa Shumaker)