In the first half of the 19th century, why did the u. s. congress work to keep a balance between the number of slaveholding states and free states?

Which four excerpts from the poem "a political reverie" by mercy otis warren imply that america was an oppressed and exploited nation? no despot here shall rule with awful sway nor orphan's spoils become the minion's prey; no more the widow'd bleeding bosom mourns, nor injur'd cities weep their slaughter'd sons; for then each tyrant, by the hand of fate, and standing troops, the bane of every state, forever spurn'd, shall be remov'd as far as bright hesperus from the polar star; freedom and virtue shall united reign, and stretch their empire o'er the wide domain. on a broad base the commonwealth shall stand, when lawless power withdraws its impious hand; when crowns and sceptres are grown useless things, nor petty pretors plunder here for kings