make or break be the factor which decides whether something will succeed or fail.

☞ A variant of this phrase, found chiefly in British English, is make or mar. The use of make together with mar is recorded from the early 15th century, but since the mid 19th century break has become more common.

2014CNN transcripts You know, big storms can test mayors, can make or break them. Imagine having to deal with it on your third day.