The unit-rule system for awarding electoral votes was firmly in place by the 1830s, put in place often by state’s majority party wanting to boost its favored candidates. Previously, states varied widely in how they selected their presidential electors, and several states went back and forth between selection by the legislature and some form of popular election. Indeed, when Thomas Jefferson was first elected in 1800, only two states used the unit rule.
Partisan calculations have been the sole driving force behind moves to change to and from the district system, particularly in large states where splitting up the electoral votes would benefit the minority party.
Virginia 1800 - Dividing the electoral votes of a state in an environment in which other states follow the unit rule weakens that state in presidential elections and disadvantage the majority party. This is why Thomas Jefferson wanted Virginia to shift from the congressional district to the unit rule system, which it did in the 1800 elections. All states that began with the district system sooner or later switched to the winner-take-all method for their electoral votes.
Michigan 1890 - Michigan had always used the unit rule since its first presidential election in 1848. But when Democrats gained control of the legislature in 1890 (the state was traditionally Republican), they enacted the district system in the hope that their candidate would pick up a few electoral votes in the 1892 election. They were right – Grover Cleveland picked up five of Michigan's fourteen votes in 1892. However, Republicans won back their legislative majority in the next round of elections and made the state revert to the unit rule.
Nebraska, 2007 - Nebraska had used a congressional district method since 1992, as had Maine since 1972. These are today the only two states to use a method other than winner take all, and no candidate has ever managed to split the vote. The use of the system also has never raised either state up to battleground status, or caused them to get more attentions from campaigns. In fact, in 2007, a movement by the some Republicans in Nebraska sought to switch back to winner take all, to maximize their power in presidential elections.
Running the numbers. . .
If a state like California (53 congressional districts) were to adopt this method, it would make a dent of 20 votes or more (how many did Bush carry in CA in 2004?) in the Democrats’ electoral vote tally.
A similar proposal promoted by some Democrats in North Carolina (13 congressional districts) would give them 3 or 4 electoral votes.
If used nationally in 2000, this system would have elected George Bush by 38 electoral votes despite Al Gore winning the national popular vote by more than half a million votes.
For more on this subject, see FairVote's report Fuzzy Math: Wrong-Way Reforms for Allocating Electoral College Votes.