After the cohen is called up for the aliya, I often hear the congregation say something. I think it ends with hayom, but I can't quite catch it. What are they saying? Response in hebrew script preferred. Also, why?
According to the Art Scroll Siddur in Shacharis for Shabbos (page 440 in the RCA edition) this is the quote from Devorim 4:4 in order to show that those called to the Torah are being blessed because they are completely loyal to it. This connection that is continuing is shown by the fact that we continue to call those who are being brought for the aliyos. Rav Hirsch explains the connotation of הדבקים
ד וְאַתֶּם הַדְּבֵקִים בַּיהֹוָה אֱלֹהֵיכֶם חַיִּים כֻּלְּכֶם הַיּוֹם:
4 But you who cleave to the Lord your God are alive, all of you, this day.