When an individual davens, he is standing before HaShem, and he should therefore be dressed respectably.[1] The Shulchan Aruch[2] writes that this includes covering one’s legs when this is how people dress in front of important individuals. The Mishna Berura[3] adds that one should wear a hat, explaining that this is the way normal people dress publicly. (In some circles, this is still true; in others, it does not apply at all.) The Shulchan Aruch[4] explains that since davening corresponds to bringing korbanot, one should have nice clothing for the occasion, just as a kohen does for his service.
The mishna[5] maintains that a pocheiach (one who is inappropriately dressed) may not read Kri’at HaTorah, serve as a chazan, or perform Birkat Kohanim.[6] When is one defined as a pocheiach? The Rambam[7] says that it is when his shoulders are exposed, and the Shulchan Aruch[8] rules that it is when one’s clothes are so torn that his arms are exposed. This ruling generated discussion regarding if this limits someone from serving as a chazan while wearing a short-sleeved shirt.[9] We accept the approach that the definition of inappropriate dress depends on whether one would dress that way before an important person (whom he sees regularly), and in most of our communities, serving as a chazan while wearing short sleeves[10] is permitted.[11]
In contrast to this, the broad consensus of poskim (that fits with the societal norms of our communities) is that wearing shorts is considered underdressed for any semi-formal setting, and such dress is thus unacceptable for a chazan.[12] Therefore, your community and you, as its agent, have every right to choose for a chazan only those who are wearing long pants.
What if the community wants to allow chazanim to wear shorts? Rashi[13] says that the problem of pocheiach (presented explicitly in regard to duchening, but probably also for a chazan) is the matter of k’vod hatzibbur (the honor of the community). One can thus claim that if the community waives any complaints, it is permitted. However, the Tiferet Yisrael[14] asserts that the limitation is not a matter of showing respect to the community, but of the community as a group showing respect to HaShem. It is important to recognize what public tefilla is. Instead of going about approaching HaShem individually, we join together, and this is expected to have a greater impact.[15] It is therefore not surprising that the halachot of choosing a representative to lead the “delegation” are quite exacting.[16] Our choice of a representative should send the appropriate message, and his being dressed in a manner that is at least presentable in the higher echelons of society is an important factor.
An argument can be presented that when the whole group shares a common shortcoming, having a chazan with the same shortcoming does not send the wrong message.[17] In particular, one might reason that if (almost) the whole group is dressed in shorts (which happens not infrequently in camp or on a trip), then even if society as a whole does not view this as respectfully dressed, the chazan might be allowed to wear shorts, as this is the standard dress for this particular group in this circumstance. Nevertheless, this is not ideal and should be avoided when there are capable candidates to serve as chazan who are wearing pants.
Of course, it is crucial to present the matter sensitively to your minyan. Nevertheless, people have a responsibility to respect the practices of a community, all the more so when this is the standard halachic indication. The mishna does distinguish between different parts of the tefilla, so it might be permitted and wise to let such a person be the chazan for P’sukei D’Zimra.[18]