The idea of Manifest Destiny, we can argue, contributed to the democratization of the United States because it stressed the importance of democracy.
The idea of Manifest Destiny was the idea that the United States had been destined by God to expand its territory. It had been destined to do this because it was superior to other civilizations. It had a good, democratic form of government. It had good, Protestant religion. It had a superior racial stock since it was built on “Anglo-Saxon” blood. For these reason, the US deserved to expand across the continent and even beyond.
This ties into democratization because of the stress placed on democracy. Our democracy was one of the things that made us better than Mexico and Canada. It was one of the things that made us superior to the Indians. If democracy was one of the main features that made us different and better, then it was important to have more democracy.
In this way, the rhetoric of Manifest Destiny helped to promote democracy. It did so because it made it clear that democracy was an important part of what made America superior.