The most famous unspoken Democratic presidential candidate alliance is probably the one between Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (D-Mass.). But there might be one brewing between former Vice President Joe Biden and South Bend, Indiana, Mayor Pete Buttigieg, as well — at least on Biden's end.

Biden and Buttigieg aren't old friends like Warren and Sanders, but there are some logical strategic reasons for why that might be the case. Biden hasn't really gone after the 37-year-old mayor very much since the primaries began, despite their ideological overlap and the fact that many of the other candidates seem to find the political upstart a tad annoying.

When asked about Buttigieg during a bus tour in Iowa on Sunday, Biden skirted the question because he didn't want anyone to twist his words into criticism. He called Buttigieg "a talented guy" and said he has no negative feelings toward him. One explanation might be that Biden wouldn't want to alienate Buttigieg's more moderate base in the hopes that they could flock over to him if the mayor falters.

But their voters actually appear to be quite different, some observers note, despite some similar policy stances. Instead, Biden might view Buttigieg as someone who could help take down Warren (which is actually the case in some recent polls) and boost the former vice president's candidacy in the long run. Tim O'Donnell