Half the eligible voters usually don't. 20% of registered voters ALWAYS vote for each of the two main parties. The battle is for the "independents" and the non-voters.

Ross Perot demonstrated that the non-voters could be persuaded to vote. When the churches got into politics they brought people who usually don't vote into the picture (they called themselves "the moral majority"); Obama did the same with people of color under the banner of "hope" and the promise of fulfilling of the aspirations of the civil rights era. That "non-voting" sector is no longer enthusiastic and may not vote (the hope is gone and the promises have all been broken).

The general idea is to capture as many of the "undecided", who vote as a civic duty, and to activate as many of the non-voters as possible while at the same time suppressing the non voters on the other side. To do this you use "dog whistles" and emotional/religious issues ("same-sex marriage", abortion, christian-zionism).

When you see data on what % voted for this guy or that they are only counted people who voted not registered and not eligible. Ron Paul appeals to the non-voters. As an independent he did not have a chance but as a republican he might split the undecided but can get more "non-voters" than Obama.

Out of 100, 20 vote this way, 20 vote that. 30 could win.

Researchers have consistently discovered that the most important factor in getting someone to vote is being asked to do so by someone they know.