Some voters are more important than others.
For the presidential race, some voters live in a swing or battleground state like Ohio or Florida or Nevada where the results can go either way while most voters live in a clearly red or blue state like Texas or California where, in almost any scenario, everybody knows who is going to win. Getting an additional Democratic vote in a swing state is much more important than in a safe state, so campaigning in a swing state yields better results.
The presidential swing states in 2016 are:
Nevada, Colorado, Iowa, Wisconsin, Ohio, Florida, Pennsylvania, Virginia, North Carolina and New Hampshire.
I'm basing this analysis from Professor Larry Sabato's site, so feel free to tweak my list any way you'd like.
For control of the US Senate, only two-thirds of the states have an election in 2016 and of those that do, only some of them are toss-up states where either party can win.
The three states that, today, are toss-up states where either party can win are:
Nevada, Florida and New Hampshire
while the states where the likely winner isn't a sure thing (and thus it is more important to campaign in those states than in states like California or Utah where the party is a lock to win are)
Arizona, Colorado, Wisconsin, Illinois, Ohio and Pennsylvania
Pretty good overlap in those lists. In other words, if we can get a random person to vote Democratic in 2016, if they live in Nevada, Colorado, Wisconsin, Ohio, Florida, Pennsylvania or New Hampshire, they will help elect the next Democratic President and help elect the new US Senate Democratic Majority Leader. That's a two-fer! Those are important people. That means we Democrats should be doing everything in our power to get every single person registered and voting in those seven states as it will help us win the White House and the Senate.
To help elect a Democratic Speaker of the House, we need to focus on those relatively few districts where either party can win. According to Larry Sabato, the list of true toss-up districts are:
Arizona's 1st and 2nd, Colorado's 6th, Florida's 18th and 26th, Illinois' 10th, Maine's 2nd, Michigan's 1st, Minnesota's 2nd, New Hampshire's 1st, Nebraska's 2nd, New York's 1st, 19th and 24th, Pennsylvania's 8th and Texas' 23rd.
That means the triple-threat voters that are the most important for control of the White House, the Senate and the House are in Colorado's 6th, Florida's 18th, Pennsylvania's 18th and New Hampshire's 1st.
If we expand the map in the House to those districts that aren't a pure toss-up but might go to the Democrats if we do everything right, there are some more districts in the important two-fer states that are already crucial to winning the White House and the Senate. They are:
Nevada's 4th, Ohio's 14th, Florida's 7th and 13th, Pennsylvania's 6th and 16th and New Hampshire's 2nd.
People who live in these 11 House districts out of 435 have the opportunity to make a significant impact on electing a Democratic President, Democratic Senate and Democratic House. In other words, only about 2.5% of American voters have that much power.
Spending our resources in these 11 districts to (a) mobilize more Democratic voters (b) convince Republican leaners to vote Democratic or (c) educate the relatively uninformed about why the Democratic Party represents their views better than the Republicans will yield a better return than in the other 424 districts because every Democratic vote we earn impacts all three big races in 2016 in these 11 districts.
Again: earning Democratic votes in Nevada's 4th, Colorado's 6th, Ohio's 14th, Florida's 7th, 13th and 18th, Pennsylvania's 6th, 16th and 18th and both of New Hampshire's districts get us the most bang for the buck.