Biden is a lame duck president with NO PUBLIC MANDATE and subterranean approval rating, he got ousted by his own party before the election for being mentally incapacitated, his defense against a Justice Department inquiry is he's too old and senile, his party just got BTFO in a national election. No NATO or NATO proxies have ever launched long range missiles into Russia in living memory. How is this guy 
fit for office
STOP EATING CRAYONS AND DO YOUR FUCKING JERB. "Senile old man sends young people to WW3 out of spite"
Those mortars aren't being used to attack Russian territory. They are used to attack Russian soldiers in Ukraine. Russia is the aggressor here so to date the predominant Ukrainian behavior has been rebuffing the invasion.
The problem will arise when anyone tries to majorly harm Russia. Ukraine tried and it was silly. They have lobbed some drones but again, minimal impact and nothing that threatening. Once the USA starts arming Ukraine for total war against Russia (meaning lobbing effective missiles, bombers, and the like) to destroy critical infrastructure inside Russian territory, it's a serious problem for Russian national security that it will need to address.
If the USA intends to launch total war on Russia, via proxy or otherwise, then we have hit the next escalation point towards WW3.