The report says it is a myth to think both sides were treated the same by the IRS.
Lawmakers can promote their favored preferences and curry favor with voters.
Taxpayers affected by the mudslides will get an extra six months to file their individual returns.
The Senate tax writing panel voted to extend dozens of expired tax provisions, in what its chairman, Sen. Ron Wyden (D-Ore.), also said would be the last markup of its kind under his watch.
The former IRS official has twice taken the Fifth before the committee.
The Senate Finance Committee is marking up a so-called tax extenders bill on Thursday.
John Koskinen, the IRS commissioner, has made increasing his agency's funding a top priority.
Senators rolled out 93 amendments ahead of a Thursday tax extenders mark-up.
The Foreign Account Tax Compliance Act, or FATCA, starts on July 1.
Panel's move to take up “tax extenders” tests lobbying clout of renewable energy sector.