"In the House, Republican prospects have been buoyed by several successful rounds of redistricting, which have sharply reduced the number of competitive seats and given the Republicans a national advantage of at least a dozen seats"

This quote by Thomas E. Mann is describing the United States House of Representatives. It is suggesting that the Republican Party has achieved success in recent redistricting efforts, which has actually resulted in fewer competitive seats and a national advantage of a minimum of a lots seats for the Republican Party. This means that the Republican Party has had the ability to acquire a bigger majority in the House of Representatives due to their redistricting efforts. This could be advantageous for the Republican Party in regards to passing legislation and having more influence in the House of Representatives. Nevertheless, it might also be seen as a negative for the Democratic Party, as they might have fewer chances to gain seats in your home of Representatives.