How do we determine the conclusion? What is different between the first and last sentence? Can they both not be premises?

We can figure out the conclusion by figuring out which statement supports the other. Looking at the first and last sentence, we have the idea that showing desire to compromise will be a benefit, and that if the compromise is rejected then it will benefit the party. Looking at these two, we can see the last sentence supports the first. The fact that rejecting the compromise will make the opposition look bad supports the idea the party should offer the compromise.