The leader of Rhode Island’s Roman Catholic Diocese said Governor Lincoln Chafee is wrong to threaten to veto legislation that would put gay marriage to a popular vote. Bishop Thomas Tobin released a statement Friday calling Chafee’s veto threat ‘‘undemocratic.’’ Tobin said it should be up to citizens to decide whether Rhode Island joins the rest of New England in allowing same-sex marriage. Chafee said Thursday that he is inclined to veto legislation that would place gay marriage on the ballot as a referendum and that he wants lawmakers to pass it themselves.