Forty-six Republican senators issued a stern warning to Democrats that they will not vote for an increase in the debt ceiling, a move that could raise the risk of the U.S. Treasury defaulting on its obligations as soon as next month.

"We will not vote to increase the debt ceiling, whether that increase comes through a standalone bill, a continuing resolution, or any other vehicle," the letter, dated Aug. 10, said. "Democrats, at any time, have the power through reconciliation to unilaterally raise the debt ceiling, and they should not be allowed to pretend otherwise."

The letter is the latest maneuver in a standoff between Republicans and Democrats over how to increase the federal government's borrowing capacity to avert any payment default. Every political impasse over addressing the ceiling in the past ended without such a catastrophic conclusion, although a protracted battle in 2011 did sow turmoil in markets and prompted a downgrade in the sovereign U.S. credit rating.