What actions did the United States government take to support slavery?

While the state governments of the South are largely blamed for the promotion of slavery, the US government as a federal institution was also heavily involved, largely using legislation to support slave-owning practices.

For example, Article IV, Section 2 of the US Constitution states that:

No person held to service or labour in one state, under the laws thereof, escaping into another, shall, in consequence of any law or regulation therein, be discharged from such service or labour, but shall be delivered up on claim of the party to whom such service or labour may be due. 

In other words, the government enforced the return of runaway slaves to their owners while prolonging the distance to the border which slaves could cross in order to claim freedom. Mandatory slave patrols used poor white people to serve as scouts for runaway slaves, with the consequence for not participating being a fine or hiring someone else to take one's place; this, of course, was not something that those in poverty could afford to do, and so the patrols persisted with a group of resentful, socioeconomically struggling men with financial motivations to do their job and to do it thoroughly. These patrols prevented the successful escapes of many slaves seeking freedom outside the South. Federal commissioners got involved by creating greater incentives for keeping slaves enslaved and coerced those who opposed slavery to participate in this process under threat of imprisonment; a fugitive freed was worth five dollars, while one who was forced into slavery was worth ten. 

Additionally, the Slave Act of 1850 made it legal for a slaveowner to cross into "free" states and to capture black individuals there under the pretense that they were runaway slaves--even if they were not. There was no use of a juried trial in order to determine those individuals' fates; rather, they were simply hauled off to the South and forced into slavery. 

Overall, Mike Maharrey, of the Tenth Amendment Center, summarizes this governmental phenomenon best:

It took centralized power to maintain a system of slavery in the United States. It was the Constitution itself that cemented slavery into the American system, and it was federal power that preserved and sustained it. Nationalists used slavery to expand federal power, and slavers depended on the nationalists to protect their interests.