In 1964, Colorado voted for Lyndon B. Johnson. It would mark the last time it voted for a Democratic presidential nominee for 28 years.

Since Bill Clinton, however, the votes have been split; three elections going red and three going blue.

Much of it comes down to two historically Republican counties: Arapahoe and Jefferson.

“There was the national convention of the Democrats here in '08 and that sort of kick-started what became a very actually democratic era,” said 9NEWS political expert Floyd Circuli.

Together the two counties account for 22 percent of Colorado's vote

When looking at active voters in 2011 Arapahoe County had 70,000 Democrats and 81,000 Republicans.

Today, the gap favors Democrats by 5,000.

It’s a similar story in JeffCo. where Republicans still have a majority among active voters but the gap has closed by 10,000 voters in the same time frame.

“I frankly no longer consider Jefferson County very Republican and historically it was a strong Republican County,” said Ciruli.

Ciruli says it's three factors -- younger voters, more ethnically diverse voters and more educated voters; groups that tend to lean left.

“Sort of a theme developed, a theory, that the state had become less of a swing state and more of a light blue,” he added.

Contrary to voting trends, Hillary Clinton is losing ground in Colorado polls.

Colorado voted for President Obama in both 2008 and 2012 by a combined 6 percent.