In the USA, can a Congressman President leave the Republicans and/or Democrats and start their own individual party?
Teddy Roosevelt did this in 1912. He tried to get the Republican nomination, but it went to incumbent president (and Roosevelt's Secretary of War). In protest, Teddy walked out of GOP convention and created the Progressive Party, which got in 2nd place in the election and carried 6 states to Wilson's 40, and Taft's 2. But this shows the risk for any party split - both halves lose and let the original oppositition party win. Some may say the same happened in 2000 and 2016, that excessive third party support cost the ruling party the presidency.
