Each Senate district in Montana is made up of exactly two House districts. House terms last for 2 years, so each member has to be re-elected each cycle. Senate terms are staggered in 4 year terms, so every two years one half - or 25 - of the districts have elections.
Another way to look at that is to say that the entire state votes for the House every 2 years, but
only half of the state votes for the Senate each 2 years. The question is are the halves that alternate in Senate elections equally distributed between Republicans and Democrats? The answer is no. In 2004, more Democrats got to choose a Senator than Republicans. The proof is in the data that came from Secretary of State's
roster (PDF) for the MT 59th Congress which show the breakdown by district and party.
25 Senate districts were up for grabs in 2004. While those districts were electing 25 Senators, they were also electing 50 House representatives - two in each Senate district. Of those, 29 elected Democratic House members while only 21 elected House Republicans. Since the House is split 50/50, that means that 29 out of 50 districts that elected Democrats
also got to cast a vote for a Senator, while only 21 out of 50 districts that elected Republicans got the same nod.
If you get that, you probably already understand that there were 25
uncontested Senate seats, and the 50 House districts that comprise those districts split with a Republican advantage 29-21.
Let's put that into plain English. Stick 100 people in a room. Half are Democrats, half Republicans (representing the 50/50 split in the House). Of those 100 people, only 50 get to vote for a Senator: 29 Democrats and 21 Republicans. Since it takes 2 people to elect one Senator, this margin of 8 people is an advantage of
four Senators (it doesn't translate literally into 4 senators, since Senate districts can split between House parties, although soon I'll show such splits favored Democrats in 2004 as well). This advantage is completely arbitrary, in that it doesn't represent a political shift. If the
other 50 Senate seats had been up for re-election instead, the Republicans would have enjoyed the advantage.
This advantage is completely different from where district lines are drawn. Instead, it exercises its power from determining
which districts will be contested
when.
It's the election schedule, stupid.
House Districts In Contested Senate Districts:Democrats (29): 4, 12, 13, 20, 31, 32, 38, 41, 42, 43, 48, 51, 52, 54, 65, 66, 73, 74, 75, 76, 81, 82, 85, 86, 91, 92, 93, 94, 99
Republicans (21): 3, 5, 6, 11, 14, 17, 18, 19, 35, 36, 37, 44, 47, 53, 67, 68, 71, 72, 89, 90, 100
House Districts In Uncontested Senate Districts:Democrats (21): 8, 15, 16, 21, 22, 23, 24, 25, 26, 33, 34, 40, 58, 64, 78, 79, 80, 95, 96, 97, 98
Republicans (29): 1, 2, 7, 9, 10, 27, 28, 29, 30, 39, 45, 46, 49, 50, 55, 56, 57, 59, 60, 61, 62, 63, 69, 70, 77, 83, 84, 87, 88