Printing money is not equal to infinite resources, you can only abuse it so much before the whole economy crumbles. And to attack Bitcoin you have to constantly burn electricity, and the only thing you really achieve is denial of service that has no lasting network effects. So, a 51% attack would only cause some short-term panic and inconvenience, which is hardly enough to kill Bitcoin. Also, with Lightning you can keep using Bitcoin even if the blockchain is being attacked by mining empty blocks, because you only need on-chain transactions to open and close channels which shouldn't be needed often.
Exactly, I don't get why some people think that just because you have a machine that prints dolla-dolla bills, you are now suddenly "printing infinite electricity" as well. You can allocate resources via your printed money up to a point, at the end of the day resources are scarce, and it would be noticeable that the government is fucking around too much.
I believe they are aware of this already. So they can only hope to bribe miners to perform an 51%, but as it has been said, they are attacking their own cash cow, they are shooting themselves in their own feet, pretty dumb to accept the bribe when you can sit on your mining machines and keep making money.
So their angles of attack are: Supporting forks, manipulation of public opinion and bribing miners to support their forks.