In what two ways does the American electoral system tend to promote a two party system?

1. The American electoral system tends to promote a two-party system because the prevalence of single-member districts and also Republicans and Democrats regularly act in bipartisan way and find common ground and work together there.

Note: This can be found in chapter 5 section 2 on page 120 under 'The Electoral System' of the The California Prentice Hall Magruder's American Government book by William A. McClenaghan.