What in the world happened in the 1948 House elections?

[Southern Senator North Carolina Yankee]

Dewey wasn't directly connected to the GOP in the Congress, so he ran better than they did downballot. Truman's attacks were mostly focused on the Republican Congress, which he lambasted as a "Do-Nothing". The attack stuck, and the people responded with a populist backlash against the Congressional Republicans.

Also, unions probably rallied against the Republicans who had led the drive for Taft-Hartley. Truman of course vetoed that, which was in turn overidden. Republicans had won a rather large number of unionized districts in 1946, leaving the Congressional Democrats heavily dominated by Southerners in non-union districts, making overide possible. The later was beyond their reach in primaries and such, but since Truman had vetoed it and Republicans were within their reach, they were the ones to suffer their wrath.

Theoretically, Obama could have tried a similar strategy. However, as evidenced by just how close Dewey came, losing to Dewey even as the Republicans lost congress to the Democrats was certainly possible. 100 years before that very thing happened to the Whigs in 1848. Obama decided not to take that risk and at points has damaged Democratic efforts down ballot (Justice dept decisions regarding VRA, fundraising moves etc etc). Winning it all is still possible in 2012, just not by virtue of this 1948 strategy.