WASHINGTON—Borrowing by the federal government is set to top $1 trillion for the second year in a row as higher spending outpaces revenue growth and concern about budget deficits wanes in Washington and on Wall Street.

The Treasury Department said Monday it expects to issue $814 billion in net marketable debt in the second half of this calendar year, bringing total debt issuance to $1.23 trillion in 2019. That would represent a slight decline from borrowing in 2018, when the Treasury issued $1.34 trillion in debt—more than...