This
article from the New York Times does a great job of showing what's at stake in this election.
McCain:
He announced an advisory committee on the courts headed by Theodore B. Olson, a leading conservative lawyer and former solicitor general, that is full of people like Charles J. Cooper, who had been influential in selecting reliable conservative nominees in the Bush and Reagan administrations.
In an interview, Mr. Olson said he was confident that Mr. McCain’s nominees would be carefully screened to assure that they were in the mold of reliably conservative recent Supreme Court appointments like Chief Justice Roberts and Justice Alito.
Obama:
In explaining his opposition to the Alito nomination, Mr. Obama said that Judge Alito’s record showed “extraordinarily consistent support for the powerful against the powerless” and “for an overreaching federal government against individual rights and liberties.”
On March 3, Mr. Obama spoke of what he would look for in a Supreme Court justice and held up Earl Warren, a former governor of California and the former chief justice, as an example.
In short, McCain vs. Obama = Alito vs. Warren.