Would the US really risk its name and face ignoring their biggest and most powerful military alliance? Honour and keeping your word are very important in geopolitics, especially among countries that have been allied for almost a century, and you won't remain the top dog very long if you avoid your duties at the first difficulty.
So yes, if Russia attacks Latvia, you better believe the US is gonna send everything they have against Russia. That's the whole point of NATO.
I'm pro NATO, but I don't think the support of the US is guaranteed. The US has always been an unreliable partner: it depends on who's in power.
If the republican war hawks or moderate Democrats are in power they would support Latvia, but libertarians and far leftists would say "it's not our problem".
There are no libertarians or far left politicians in the Oval Office, now or in the near future.
I cannot speak to how the US would behave in 50 years, but being true to its word it is mandatory for the US to project itself as the "world leader". The day the Oval Office is able to ignore its allies, it is the day the US is no longer a global leader.
That's simply the cost to pay to be seen as the leader of the free world. The fact that some US politicians openly want to go down the route of reneging its allies speaks volumes about how long the US empire will last.
Is Trump a libertarian or a far leftist? Because he's currently the cause of these discussions -- he's the one that people have the most doubts would agree to defending Latvia.
He's neither. He's a big State conservative populist that cozies up to the far-right cretins as they represent what a lot of the silent, dissatisfied majority thinks.
So yes, if Russia attacks Latvia, you better believe the US is gonna send everything they have against Russia. That's the whole point of NATO.