charm offensive 
noun
A campaign of flattery, friendliness, and cajolement designed to achieve the support or agreement of others: he launched a charm offensive against MPs who didn’t support the government...
- It was quite a charm offensive by the seasoned campaigner.
- He left, having accomplished what I later realised was a retrospective charm offensive, aimed at persuading me that his show of aggression was an aberration.
- The Prime Minister will today launch a charm offensive on the party's grass roots activists, praising them as the ‘biggest heroes’ in the party's first 100 years.