In swing states, the share of the vote that goes to the incumbent party is about 50 percent, and economic and other factors can swing the vote to either party. The key swing states for 2016 include Colorado, Florida, Ohio, Virginia, Iowa, New Hampshire, Nevada, Pennsylvania and Wisconsin.

Voter fatigue could weigh heavily against the Democratic nominee. Several swing states would be inclined vote to for the Democrat if not for the two terms President Obama has spent in the White House, Moody’s said.

The direction of president’s approval rating could also make a meaningful difference.

If Obama’s approval rating improves leading up to the election, he will probably give his party a boost. But in most elections, the president’s rating has declined in the lead-up to the election, favoring the challengers.