turn of the tide
A reversal of fortune, as in This last poll marked the turn of the tide, with our candidate gaining a sizable majority. Similarly, to turn the tide means "reverse a situation," as in The arrival of reinforcements turned the tide in the battle. This idiom transfers the ebb and flow of the ocean's tides to human affairs. Although the idea is much older, the precise idiom dates from the first half of the 1800s.