WASHINGTON—The U.S. Treasury Department estimates it will issue more than $1 trillion in debt this year as higher government spending and sluggish tax revenues push the deficit higher.

The Treasury said Monday it expects net marketable debt to total $425 billion in the fourth quarter, which would bring total debt issuance in 2018 to $1.338 trillion, compared with $546 billion in 2017. That would be the highest annual debt issuance since $1.586 trillion in 2010, when the U.S. economy was still crawling out of a recession.

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