I agree with Peter. As for your follow-up questions, "mobilize" is a word commonly used in politics, which in that context means to take political action, particularly within a particular group. If you "mobilize" a group of voters, it's like getting them to go out and vote or maybe getting them to go out and campaign or take other political action. "Proportion" refers to the percentage of people. So in this case, as Peter said, white evangelicals are a small percentage (or "proportion") of the population, only 15%. However, they have "mobilized" themselves to be very politically active, and to make up a much higher percentage/proportion of the electorate (aka, people who vote) at 25%. So they have mobilized themselves to be a higher proportion among voters compared to their proportion in the population.

The contrast is between the percentage of this group in the population versus its representation among voters. According to this report, white evangelicals have shrunk as a percentage of the population. Their proportion among voters has remained steady, however. The implication is that more of them are voting even though there are fewer of them.

I would love to know history of you and your family.