Does the idea of Manifest Destiny unite us or divide us in America today?
Today, the idea of Manifest Destiny (to the extent that it exists at all) divides us.
In the past, almost all Americans agreed that they were racially and culturall superior to Indians and to Mexicans and that they deserved to take the lands those peoples owned. Today, there is much less consensus on this idea.
You could say that the arguments over whether we should use military force to impose our will on others is an argument about Manifest Destiny (I don't think it is, but it is possible to make the argument). Thus, when George W. Bush says that we need to invade Iraq in order to make them democratic, it is an appeal to Manifest Destiny. However, there are many people in the US today who do not believe that we should try to do this sort of thing. There is no longer a consensus on this idea and it serves more to divide us (think about what Democrats said about Bush's foreign policy or what some Republicans say about Obama's alleged "apologies" for America) than to unite us.