Walrus, on re-reading your post, I thought it might be helpful to clear up the mention of "seats". The election under discussion has nothing to do with seats in the legislature. It is strictly for selection of the president. The legislature does not routinely elect the president, although the job can fall to the legislature if the normal Electoral College process fails in certain ways. It's quite rare for that to happen (twice in 200+ years, I believe), but there's some chance it could happen this go-round.

The legislature is elected as follows:

Senators, who represent the state as a whole, are elected by statewide vote. Each state has 2 senators, serving in the Senate.

Congressmen are elected by a portion of the state in a voting area called a Congressional District. The number of congressmen varies with the population of the state (some states have but one, Florida has 23, California has--I dunno, a whole bunch). Congressmen serve in the House of Representatives.

The NUMBER of presidential electors from a given state equals the NUMBER of senators and congressmen combined (in Florida, 2+23=25). However, they are not the same people. Presidential electors perform their function only in a presidential election, every four years. Senators and congressmen, of course, do their job throughout their terms.