Well, there are many many bruises to the constitution over the years from supposedly well intentioned laws and seemingly constitutionally sound laws .... all leaving the thing relatively defenseless.
...A couple of major assaults ..
A ... Universal suffrage - Probably the first huge chink in a logical approach to governing. When a very very large percentage of people without property and with great dependence on government support hold the right of suffrage - they are automatically in place to damage the hand that feeds them. This is the "problem with democracy" that Jefferson as well as Adams feared so much. The REPUBLIC model was supposed to be a check against the rabble of congress (the people's voice). Jackson was an early proponent of 'populism' which started the ball rolling toward universal suffrage. It has gone so far as to allow "democratic populism" to override fiscal responsibility. Too late to fix - but a selective suffrage system not based on color or sex or religion - but on property and tangible contribution to the national treasury or to the national defense -- would go a long way to sound policy for all.
B... The destruction of states power by changing Senatorial selection from the domain of state legislatures to the domain of popular vote within the state. This opened the door for national interests in regional affairs of elections. Today's senators are not beholden to their states in any meaningful way as were the Senators before 1900. They are tools of large global or national interests with little to hold them on track with the wishes of their own state's citizens. When senators were chosen from smoke filled rooms of state legislatures - they were powerfully beholden to the interests of their state and the people THERE - who took their legislators to task in state elections - not national elections affected by massive funds from areas of interest far from local concern.
so sayeth uncle vic