The U.S. Treasury's relentless expansion of T-bill issuance is blurring the line between fiscal management and monetary operations, effectively stepping on the Federal Reserve's turf.

On September 2, Bloomberg macro strategist Simon White warned that this trend could structurally amplify inflationary pressures, undermine the Fed's policy independence, and heighten risks to market stability and real returns on both stocks and bonds.

T-bills now account for 22.7% of the nation's outstanding debt—exceeding the Treasury's own informal 20% ceiling. Excluding Fed holdings, that figure climbs to 24.1%.

White argues that as fiscal deficits keep widening, this ratio is poised to rise further. At that point, the policy rate will no longer be just a lever for monetary policy; it becomes a critical anchor for fiscal sustainability.

This structural shift means the Fed will face stiffer political and fiscal headwinds if it tries to hike rates to tame inflation. Every rate increase directly raises government borrowing costs, effectively tightening fiscal conditions on autopilot—creating a feedback loop that pressures monetary policy in the opposite direction.

White's blunt assessment: with T-bill issuance expanding, finding an interest rate that simultaneously hits inflation targets, maintains financial stability, and keeps government borrowing costs under control is—in his words—perhaps an impossible mission.

From financing tool to shadow currency: the evolving role of T-bills

T-bills have long been a routine tool for short-term government financing. But White contends the Treasury is deliberately substituting T-bills for long-term bonds to absorb the funding needs created by expanding deficits.

From the Treasury's own perspective, the logic is clear: short-term debt typically costs less than long-term debt, and T-bills tap into a broad pool of investors who prefer low-duration assets. Compared to long bonds, T-bills also create less of a drain on bank deposits, helping to preserve market liquidity and support real-economy spending. Additionally, this path partly sidesteps the political sensitivity of overt financial repression—even though the Treasury's expanding buyback program is already seen as a concrete step in that direction.

In White's view, however, T-bills have outgrown their role as mere short-term instruments. Drawing on Perry Mehrling's money hierarchy theory, the top of the financial system is occupied by Fed reserves, followed by bank deposits, repurchase agreements, and money market fund shares—collectively termed "shadow money."

T-bills, given their ultra-short maturities and minimal valuation uncertainty, typically enjoy a zero haircut in repo markets. They can also be re-used repeatedly through collateral rehypothecation without losing value.

White notes this brings T-bills functionally ever closer to money itself—a liquidity instrument usable for final settlement.

Rehypothecation loops amplify a shadow money supply expansion

White zeros in on the critical role of rehypothecation in this process.

In repo markets, dealers can source collateral from their own Treasury holdings or borrow via reverse repos, then re-pledge the latter to other counterparties. According to a 2021 academic study cited by Bloomberg, between 2015 and 2021, U.S. Treasury collateral was rehypothecated three to five times on average; in other periods, some estimates put that number even higher. In effect, dealers are continuously multiplying the effective supply of collateral available in the market.

For long-term Treasuries, White points out, each round of rehypothecation gradually erodes collateral value through haircuts. But T-bills—with zero haircuts—can be pledged repeatedly without any loss of value. This trait lets T-bills replicate themselves throughout the financial system, creating a money-like liquidity expansion effect.

Historically, a rising T-bill share of total outstanding debt has often led structural inflation with a noticeable time lag. White sees a plausible transmission channel: increased liquidity lifts asset prices, strengthens the wealth effect, and artificially suppresses capital costs—eventually feeding through to the real economy and pushing up overall price levels.

The Fed's dilemma: policy independence under fundamental challenge

White argues the expanding T-bill issuance imposes a fundamental constraint on the Fed's policy space.

When more public debt is directly tied to short-term rates, every Fed hike immediately raises government interest expenses—effectively an automatic fiscal tightening. This mechanism creates objective political pressure, making it harder for the Fed to act decisively when inflation flares.

White is direct: as the T-bill share keeps rising, "the Fed may no longer be able to set policy in an optimal way to achieve its inflation target."

Meanwhile, a larger T-bill supply could also inject fragility into short-end funding markets. If new issuance pushes T-bill yields up, money market funds may shift cash from repo into T-bills, draining repo market liquidity.

White specifically flags the current backdrop—where the Fed's reserves plus reverse repo facility, relative to GDP, sit at historically low levels—as making the risk of funding market stress from such a migration particularly acute.

On the financing structure front, the Treasury itself faces greater vulnerability. More T-bill issuance means more frequent and larger auctions, and any uptick in inflation expectations quickly feeds into borrowing costs. Even if the Treasury tries to trim long-term issuance, a sharp rise in term premia could still push long-term financing costs higher in the end.

White's conclusion is decidedly pessimistic: as the T-bill share expands, the policy rate will be forced to carry the dual burden of monetary policy and fiscal stability. And finding a single equilibrium rate that satisfies inflation targets, financial stability, and fiscal sustainability all at once is, in all likelihood, an unsolvable puzzle.