What federal legislation passed in 1850 would allow law enforcement officials to capture runaway slaves?

The Fugitive Slave Act.

It reflected the increasing difficulty of creating new slave-states during the Western expansion.

As the Compromise of 1850 mostly favoured the North, Congress had to make a big gesture of appeasement to the South. It said, in effect, "We can't offer you much hope of new slave-states, but my goodness, we'll crack down on those runaways."

It backfired badly. Northern citizens resented being treated like unpaid slave-catchers, and 'Uncle Tom's Cabin' was written as a protest against the Act.