@Feniks, don't worry, it was difficult and pawian's hints were somewhat confusing, imho :))
PiS strongly argues that Poland 2050 and PSL will fall short of the threshold, but they (PiS) can go on a ride with it.
I think it's the idiom "przejechać się na czymś/kimś". Which means that, for example, you count on something to happen, but you can get severely disappointed. So, PiS strongly argues that Poland 2050 and PSL will fall short of the threshold, but it can turn out that PiS will get very disappointed, because it may not happen.
It's the second meaning:
sjp.pwn.pl/slowniki/przejecha%C4%87%20si%C4%99.html