From 1860 to 1930 the Republican Party controlled both (usually) or at least one of the houses of Congress except for 4 years during the Wilson administration from 1912 to 1916 when Democrats managed to gain majorities temporarily.
Then, in 1932 in what political scientists call a realigning or earthquake or critical election, Democrats gained the presidency, both houses of Congress and majorities in state legislatures and governorships. Except for brief 2-year periods, the Democrats maintained that control of both houses of Congress till 1980, despite the election of Republican presidents Eisenhower and Nixon.
Since 1980, neither the Republicans nor the Democrats have been able to keep majorities in Congress for longer than 12 years max, and often the president was a member of the party opposite to the one that controlled Congress. Neither party has held the presidency for more than 8 years at a time other than George Herbert Bush for 4 years after Reagan’s 8 years.
This situation is what characterizes the 6th Party System as described in your book and is what political scientists call “divided government”. Since the 2000 election, the popular vote for president has been less than 3 ½ % points apart except for the 2008 Obama win which was a 7% – point difference. Only 10/44 elections prior to 2000 were within that 3% range of popular vote difference.
Even leading up to the Civil War from the 1848 to the 1860 elections, the % difference between the winning candidates was larger than 3%.
In addition to the close division of Americans between the parties, there are also deep divisions within the parties themselves. An example of the effect of those divisions is the fact that although the Republican Party held majorities in both houses of Congress during 2017 and 2018, they were unable to present and vote on a health care plan or repeal or “tweak” the ACA (Obama Care).
Currently, the Democrats have a majority in the House, but the division in the Senate is 50-50 with one Democratic Senator who leans toward the Republican side of many issues due to his constituency in West VA. So, the Democrats are also having difficulty passing bills that are part of their initiatives. (For a look at the numbers of bills introduced versus those getting passed from the 1970s up to now, see this site: Historical Statistics about Legislation in the U.S. Congress — GovTrack.us Congresses now are doing about ½ of what the institution accomplished in the 1970s and 1980s).
So, divisions between the parties AND divisions within each party are complicating governance…dealing with the issues that the American public needs answers to.
Look at the factions within each party as set out in your book on pages 320-321.
Pick a party.
Accept the reality that compromise must happen and every faction is not going to get everything it wants.