The Senate vote (on March 19, 1920) was 49 yea, 35 nay. Of the nays, only 12 were Republicans, the other 23 were Democrats.
I haven't found corresponding figures for the yeas, but that particular Senate was Republican by 49/47, so even supposing that all 24 remaining Democrats voted yea (in reality some probably abstained or were absent), that still leaves 25 Republican yeas. IOW, Republican Senators voted yea by at least 25-12 - more than the required two-thirds majority, so that had Democrats voted yea in the same proportion as Republicans, the Treaty would have passed.