Whether or not potential candidates have access to significant financial support influences their decisions to enter the race. It is extremely expensive to run a competitive campaign because of costs associated with staffing, travel, advertising and more. But candidates who fare well in the early contests tend to raise more money and survive longer in the primary process.

The mood of the electorate also influences potential candidates’ decisions about whether to run. If the incumbent president is very popular – a rarity in modern American politics – it may scare off some would-be challengers.

But the public can be fickle. An incumbent may be popular a year before the general election, just as George H.W. Bush was in early 1991, only to see their popularity fade the following year. Bush lost the election to Bill Clinton in 1992.

The political fortunes of unpopular incumbents also can shift. In 1983, Reagan’s favorability ratings were very weak, but he rebounded by 1984 and beat Democratic candidate and former Vice President Walter Mondale in a 49-state landslide victory.

During presidential election years when there is no incumbent, as in 2008 and 2016, potential candidates’ calculations don’t have to include incumbent popularity. In 2016, both Trump and Sen. Bernie Sanders, an independent from Vermont who sought the Democratic nomination, were able to tap into an electorate looking for change by appealing to supporters with populist messages.

Trump’s effort successfully secured the Republican nomination, while Sanders’ effort came up short as the Democratic party favored its first female nominee, former Sen. Hillary Clinton.

From determining whether an incumbent president is vulnerable to a challenge from within the party to the likelihood of defeating an incumbent of the opposite party, a significant amount of strategic planning is involved in any effort to win the presidency. And the planning begins long before the day candidates announce their intention to run.

This article is republished from The Conversation, an independent nonprofit news site dedicated to sharing ideas from academic experts. If you found it interesting, you could subscribe to our weekly newsletter.

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Robbin Mellen Jr. does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.