WASHINGTON, Aug 21 (Reuters) - U.S. lawmakers will aggressively question U.S. Postmaster General Louis DeJoy in his first appearance before Congress on Friday about recent cost-cutting measures that Democrats say appeared to be an attempt to boost President Donald Trump's re-election chances.

Under pressure from the public and lawmakers, DeJoy on Tuesday suspended all mail service changes until after the Nov. 3 election. Critics feared they would interfere with mail-in balloting, which is expected to be much more widely used amid the COVID-19 pandemic.

Trump has repeatedly and without evidence said that an increase in mail-in ballots would lead to a surge in fraud, although Americans have long voted by mail.

The Republican chairman of the Senate committee holding Friday's hearing, Ron Johnson, will defend DeJoy in his opening statement, citing his "commendable attempt to reduce those excess costs that are now being cynically used to create this false political narrative."

Democrats will want to know whether DeJoy plans to undo changes to the mail made in recent weeks. Changes that threatened to slow mail delivery - and in some cases, already have - include reductions in overtime, restrictions on extra mail transportation trips, and new mail-sorting and delivery policies, enacted in an attempt to cut costs.

DeJoy, who has also agreed to testify before the Democratic-led House Oversight and Government Reform Committee on Monday, will testify before the Republican-led Senate Homeland Security and Governmental Affairs Committee on Friday.

While criticism from Democrats is expected, any signs that Republican senators are unhappy with DeJoy's cost-cutting efforts could suggest his tenure as postmaster general is at risk. DeJoy, a major political donor and ally of Trump, assumed the job in June.

A group of 90 Democrats in the U.S. House of Representatives on Wednesday called on the Postal Service's board of governors to immediately remove DeJoy "to protect this critical institution," according to a letter sent to board members.

The House is set to vote on a bill on Saturday that would provide $25 billion in funding for the Postal Service and require the reversal of operational changes. (Reporting by Chris Sanders; Editing by Peter Cooney)