Lawmakers have been working to strengthen regulation of non-traditional compounders since the New England Compounding Center, a Mass.-based firm, caused a deadly outbreak of meningitis last fall.
The Harkin-Alexander bill has advanced the furthest, unanimously passing the HELP Committee on May 22.
In their letter, the two senators pointed to a new string of infections tied to products from a compounding pharmacy in Tennessee.
The lawmakers also cited warning by federal health officials that "if we fail to act, this type of incident will happen again."
The legislation would require the Food and Drug Administration to regulate non-traditional compounders, which produce drugs on a large scale and ship them across state lines. State regulators would retain oversight of small, community-based compounding pharmacies.
The House has considered a similar move, but no bills have advanced through committee.