Reapportionment reflects the population of a state by dictating the number of representatives and their political affiliation in Congress. Reapportionment also affects the number of electoral votes a state has in a presidential election.
Every U.S. Census determines the amount of congressional districts and electoral votes a state is entitled to based on population. States with decreasing population sometimes lose congressional districts, which requires realignment of the existing districts.
Since the number of districts in each state determines the number of representatives in the House of Representatives, losing districts also means losing representation in Congress. The opposite happens for states that experience a significant gain in population.
A district's political affiliation is also affected by reapportionment. If a district with a historical attachment to one party realigns with a district loyal to another party, the change in affiliation can affect state and federal elections.
Finally, population directly affects the amount of electoral votes each state receives for a presidential election. States that significantly gain or lose a portion of their populations accordingly receive or lose electoral votes. Since states, like districts, tend to trend toward one party or another in presidential elections, the gain or loss of electoral votes can significantly impact the outcome of a presidential election.