Regarding the 15th Amendment, Wikipedia , citing Gillete (1965) , says that no Democrats in the House voted in support of the bill. It's unclear if any Democrats in the Senate voted for it, but it doesn't appear so.
Here is relevant Library of Congress  page, but it's hard to find explicit voting records (sorry, don't have time to pore through the Congressional Globe ).
Found Senate Vote 586  which shows no Democrats voting 'Aye.'
It's incorrect, I think, to say that there was 100% Republican support for 15. The record shows 3 Republican Senators voting against, and 20 abstaining. Again, Wikipedia cites Foner, 1988  as saying that some Northern Senators abstained to prevent the South from gaining too much political power. Finally, Senator Charles Sumner  abstained (according to Gillette via Wikipedia) because the Amendment did not forbid poll taxes and literary tests, so perhaps the abstentions cannot be viewed as lack of support for voting rights for blacks.
But of course, the Republican and Democratic parties of the 1860s and 1870s are quite a bit different than those of today. (No source for this, but it seems rather obvious that political positions are going to shift dramatically over the course of 150 years.)