WASHINGTON—U.S. Treasury Secretary Steven Mnuchin is pitching Senate Republicans on a roughly $850 billion economic stimulus package, according to two people familiar with the matter, as the Trump administration works to craft more legislation to cushion the economic blow of the coronavirus pandemic.

Mr. Mnuchin discussed the outline of the measure with Senate Republicans at the Capitol on Monday night, and he is expected to more fully detail the proposal during a broader meeting of Senate Republicans on Tuesday.

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