There is two ways to remove obama :
1. Voiding his election, this applies to a public employee(yes,obama is a public employee) . If obama is found to never have been ineligible to be President,then his election can be voided, he never ran for office, never chose biden , never made a appointment etc., never was the President. In this case McCain would be the legal President of the United States.
2. obama resigns, then biden becomes President.
If it looks like obama is going down for being ineligible , them you can bet that the “party “ will force him to resign, to make biden the President. They would never let it get to court and give the office of the President to McCain.
Biden’s selection as a candidate was technically by the Democratic National Convention, not by Obama personally. Biden has no constitutional problem. He was elected distinctly by the electors who were duly chosen by halfwit voters on election day 2008.