Sen. Claire McCaskill had to lock up the free ice cream after the hordes descended. (AP Photo/J. Scott Applewhite, File)

Among Capitol Hill staffers, no rumor travels faster than one involving free food.

So, when word got out last week that Sen. Claire McCaskill (D-Mo.) had installed a commercial-sized freezer in her office lobby stocked with gratis ice-cream treats, the resulting stampede was as predictable as a party-line vote.

But those who arrived a day late to the run on the sweets were out of luck. One staffer from a nearby office found the case locked, and was turned away, cone-less.

“While we’re pleased the freezer has generated such interest and a number of wistful, longing glances from other offices, its sweet, chilly contents are reserved for Sen. McCaskill’s constituents who are visiting Washington this summer,” her spokeswoman tells us.

Most congressional offices offer treats to vistitors made in their home states or districts (it’s impossible to leave a Georgia member’s office without a bag or three of peanuts). Ethics rules allow members to accept such goods from manufacturers back home, making the arrangement an all-around win: it’s good publicity for the companies, and members like the chance to offer guests a little something. (Hey, with Congress’ approval-rating at an all-time low, who wouldn’t want to throw in a free candy bar with that governance?).

While the snacks are most often given to folks on business visits or in tour groups, aides from neighboring offices sometimes drop by to partake. McCaskill’s ice-cream lockdown is a testament to the novelty of the offering — and the sweltering temperatures

And, it turns out, it’s only for a limited time.

The freezer in the Missouri Democrat’s digs is on loan for the month of July, aka, National Ice Cream Month, from Unilever, which produces frozen favorites like Breyers, Good Humor, and Klondike, at its Missouri plant.

So get them while — and if — you can.

And if not, there are plenty of other goodies available. Here are some of the most popular items up for grabs in Senate offices, according to staffers:

Sen. Ted Cruz (R-Tex.) — Dr. Pepper and Doritos

Sen. Kirsten Gillibrand (D-N.Y.) — Chobani Greek yogurt

Sen. Bob Casey (D-Pa.) — Peeps (seasonally, of course)

Sen. Rand Paul (R-Ky.) — Pop Tarts

Sen. Brian Schatz (D-Hawaii) — Chocolate-covered macadamia nuts