Why are there only two dominant political parties at the national level? Is the two-party political system good for American democracy?

Are there conditions that are necessary for a reform movement to gain
momentum or critical mass?

American electoral politics have been dominated by two major political parties since shortly after the founding of the republic. Since the 1850s, they have been the Democratic Party and the Republican Party.Some historians have suggested that two-party systems promote centrism and encourage political parties to find common positions which appeal to wide swaths of the electorate. It can lead to political stability which leads, in turn, to economic growth.

Ummm they were radical

Ithink it is 4. making colonies.