"Teddy Roosevelt tried it when?

This is the 21st century. Surely circumstances have changed a bit."

Not as much as you would think when it comes to politics.   Since TR there have been many attempts- most recently from Ralph Nader and a bit further back from Ross Perot and John Anderson.   It appears taht successful third parties tend to center on a specific issue and can make enormous change by influencing votes, but they have not been able to make a stand as a serious contendor to the Democrats and Republicans.