The U.S. government's attempt to alleviate the short supply of T-bills is about to get a little harder.

After years in the making, a post-crisis rule to prevent a run on the money-market industry will finally take effect this October. It will force funds that oversee about $600 billion to abandon a fixed $1-a-share price and float their net asset value (NAV). But because businesses and state governments treat the funds like bank accounts, the prospect of prices falling below a buck is causing a big shift into money-market funds that buy only government debt.

To cope, the Treasury Department stepped up bill issuance and boosted supply by almost a quarter-trillion dollars after its share of U.S. government debt fell to multi-decade lows last year. That still might not be enough. Estimates suggest between now and mid-October, the influx may produce an extra $400 billion or more in demand for short-term government securities and put a squeeze on a sizable chunk of the $1.51 trillion bill market.