Id say he would easily win, Taft would probly be his VP he left him as his successor in OTL after all. The POD could be him not promising to stay for only one turn in the prior election, he reportedly ended up regretting that decision. He likely wins handily as Taft did OTL and continues his Progressive agenda, it gets interesting when 1912 rolls around. I dont think he would run for a third term under these circumstances, the only reason he ran OTL was because of Taft's supposed betrayal of his policies. I bet we would get Roosevelt 1908-1912 then Taft 1912-1916 making for an interesting WW1, perhaps Taft decides to stay out of the conflict completely unlike Wilson's "we're neutral but still sell tons of vital war supplies to the allies" neutrality.