Aaron Blake points us to an extremely useful chart: A guide to presidential wins by congressional districts, compiled by Fun With Party ID, a blog I now have to check constantly. The blogger calculated the advantage from every election, according to contemporary congressional redistricting. Notice when the Republican advantage becomes consistent: 1992.
What happened in 1992? Something that started rolling in 1982. That was the year that a Democratic House and Republican Senate amended and renewed the Voting Rights Act, strengthening the requirements to craft majority-minority districts. Those new rules went into effect in the 1991 rounds of redistricting. In 1990, only 27 of 435 districts were majority black or Hispanic. In 1992, 51 districts become majority-minority. The result: Even though Bill Clinton won the presidency, the new districts and some scandal-based retirements gave Republicans a net gain of nine House seats.
In 2001, gerrymander gurus followed the same rules, but Republicans controlled more legislatures -- Michigan, Florida, Pennsylvania, Ohio, etc. In 2011, plus ca change. This is the less-discussed, more-disturbing aspect of the GOP's theoretical campaign to assign electors by congressional districts. Black voters, already packed into House districts that minimize their clout, would see their clout further reduced in presidential races.