The Boxer/Casey letter comes months after the senators pushed legislation through the upper chamber that would have barred lawmakers and the president from collecting paychecks if the government had shut down.
Boxer tweeted early Tuesday afternoon: "If #Congress cannot do its job & protect the full faith and credit of the United States, members should not get paid."
The letter to Geithner points out that the Senate had already passed legislation in March on the issue.
Senate Judiciary Committee Chairman Patrick Leahy (D-Vt.) criticized the bill earlier this year.
The House is expected to vote down a clean vote on raising the debt limit on Tuesday night.