Senators can't be gerrymandered because they represent the entire state. A pre-set geographic boundary which (usually) can't be changed. Gerrymandering happens after each Census (2010, 2000, 1990, 1980, etc) when the House seats are reapportioned and redrawn to be relatively equal in population.
If you want a recent Democrat example, just
look at California. In the 2014 House elections, Democrat candidates got 57.7% of the votes relative to Republicans (4.06m vs 2.98m). Yet they won 73.6% of the races (39 of 53). Of the 9 races where the winner got fewer than 57.7% of the votes, Democrats won 8, Republicans just 1.
Anyway, this is nothing new. The term Gerrymandering
dates back to 1812. Letting the State legislatures draw the election districts is literally letting the foxes guard the henhouse (gerrymandering isn't just about helping your own party, it's also about making "safe" districts so incumbents have an easier time getting re-elected). In the 1990 election, California ended up with a Democrat-controlled legislature and a Republican governor. The Democrats gerrymandered the districts, and the governor vetoed it. The boundaries ended up being drawn by the State Supreme Court, and for the next 8 years California had probably the fairest elections in its history.
There were two California ballot initiatives in 1990 for taking control of redrawing the districts away from the legislature. They were both winning until about a month before the election. Basically every special interest out there realized fairer districts would add unpredictability by increasing the chances of incumbents losing. So they all ran ads against them (including several groups I had previously thought were "honest" like the Sierra Club and NOW). And both initiatives were defeated.