Larger midterm victory: 1958 for the Democrats or 1994 for the Republicans

[Penn_Quaker_Girl]

I'd go with 1994.  

My dad talks about a time when "Democrats always held the House.  It was the Senate that was sometimes up for grabs".  

While I can't speak much to either election cycle from a personal standpoint, 1994 absolutely rocked the foundation of Congress.  Pigs had flown -- the Republicans had found a way to win control of the House (and ousted the sitting Speaker to boot).  

It was also a pretty significant cultural and tactical shift for the Republicans.  While partisanship was not a new thing, the right turned up the dial on "our enemy, the left"-type rhetoric.  The '94 Republican Revolution is a big part of the reason that, outside of 2008, I cannot envision an America where people vote on the issues and not their team winning.