Campaigns are very expensive: They need to finance candidates' travels around the country, rallies, television and digital advertising, and administering their giant campaign staffs. This money comes from other people and organizations who donate their own money.

Politicians spend a lot of time soliciting donations from regular voters, who are limited by law to donating a few thousand dollars. But much more money comes from big groups who funnel money into organizations that support candidates but are not held to the same limits, such as parties, or political action committees (PACs). For this reason, presidential candidates spend a huge amount of their time trying to get cash from these groups to power their campaigns.

A presidential candidate needs to make a compelling case for why they should occupy the White House through their campaigns. Candidates need to give voters a clear sense of how they will use the power of the presidency.

Campaigns typically involve policy experts who work with candidates at a high level to design a policy agenda they can promise voters. Of course, candidates can and do promise lots of things on the campaign trail that they can't immediately deliver. While campaign promises can energize supporters, if a president later changes their mind or fails to follow through, it can be costly for them when they come up for reelection.

Designing policy on the campaign trail needs to balance both the electoral benefits of certain promises with the potential that the policies can become costly should they win the office.

Campaigns are large, complex national organizations with professional staff of hundreds or thousands and countless more volunteers.

Managing the day-to-day operations of campaigns is an immense logistical task with many dedicated professionals making sure campaign people and resources are going where they need to be. While not the flashy work of speechwriters or strategists, the nuts and bolts of campaigns can be essential for making sure the campaign effectively gets its message out to voters.

Campaign communications, namely advertising, are the most expensive part of a campaign. Candidates spend millions of dollars on TV ads and, increasingly, digital ads on social media and Youtube.

They also think about how the media covers their campaign as a sort of advertising, using events and press releases to control the image and narrative that are projected in the news. Speechwriters help design presidential candidates' talking points that best convince voters they are the right person for the presidency.

Campaigns are also about reaching people and having conversations: convincing them that you are the best candidate, to get out and vote for you, and to contribute their time or money to your campaign. For this reason, campaigns invest heavily in "field operations" where large staffs of volunteers make phone calls and knock on doors to reach voters directly in important states or districts.

In recent elections, candidates in both parties have come to realize the pivotal importance of the "ground game." Many people believe President Obama's win in 2008 was powered by his innovative and expansive grassroots campaigning. In 2020, both Republicans and Democrats are hoping to build very large field operations.