One... don't call him Teddy. He *hated* that nickname, none of his friends dared use it to his face.
Second, I think the best way is to have a more conservative Democrat, say Clark, win the nomination. Then the progressive vote which was split between TR and Wilsn goes to TR more heavily
Clark was not more conservative than Wilson. He had been a long time Bryan supporter until they fell out at the 1912 Convention. The Tammany delegation from New York had switched their votes to Clark, and Bryan used this as an excuse to declare against him - presumably in order to deadlock the Convention in the hope that it would turn to him.
If Clark is nominated, he may lose a few Conservative votes to Taft, but can probably make them up at the expense of Eugene Debs. There is no reason for TR's vote to be much altered.
Not that it really matters. The last conservative to get the Democratic nomination, Alton B Parker in 1904, won nearly 38% of the popular vote, only about four percentage points less than Wilson in 1912. If Harmon or Underwood were nominated (very unlikely) and did about the same as Parker, and further asuming that all the four percent switched to TR (unlikely again - ex-Democrat Debs could expect to get many if not most of them) that still leaves TR on only 31-32% - still six or seven points adrift.
Sorry, but for my money it's ASB. The Democrats were "on a roll" after their 1910 landslide, and were always the odds-on favourites even in a two-way race, never mind a three-way. If the results in CA and SD (where Taft was not on the ballot) are any guide, TR would have an uphill battle even in a straight fight with the Dems. With Taft remaining in the race, he was toast. His letters from the period suggest that he himself realised this, but was so furious with Taft as to be past caring.