Romania isn't saying no to Bulgaria and Hungary only. The partition had the support of the two bullies in the block, i.e. Germany and the Soviet Union. The traditional ally of Romania had been France, and by the summer of 1940 it was in no shape to provide help. Britain had offered a guarantee, but had other fish to fry. Italy had been a possibility, but not now that it was an ally in the war with Germany.
So if Romania says no, chances are it gets invaded, from all sides save Yugoslavia. The Hungarians have German support, with Hitler being worried that the Soviets might get too close to Ploesti.
What happens then? Hard to say. Romania can't win, but one wonders how long this will drag on, and with what consequences, especially on the starting positions for Barbarossa the following year.
And now for a bit of shameless self-promotion:
GURPS WWII: Michael's Army.