By the 1850s the constitution originally framed as an instrument of the national unity, had become a source of sectional discord and tension and ultimately contribute to the failure of the union it had created.
assess the validity of this statement

This is of course a somewhat subjective question, but most would agree that this is true, since the failure of the Founding Fathers to address the issue of slavery in the Constitution meant that no amount of "compromises" could work to fix the issue of incorporating slavery into new states, which ultimately led to the Civil war.