First of all, what's your evidence that the Dominican people supported annexation--Baez's ridiculous rigged plebiscite (the fact that the vote was 15,169 to 11 should by itself be enough to draw suspicion)?
http://books.google.com/books?id=OsTffvilQWgC&pg=PA139
Second, I don't think the treaty ever had a chance in the Senate, at least once Sumner came out against it. The Senate's June vote on the treaty was 28-28, well short of the two-thirds majority needed for approval. It is true that the Senate later voted 32-9 to establish a commission to study Dominican annexation. However, "Many senators voted aye not because they supported expansion but because the commission proved a face-saving way for Grant"--and a rebuke to Sumner, whose moralistic carping had become annoying even to those who agreed with him that annexation was unwise..
http://books.google.com/books?id=9-alDgRk-IYC&pg=PA165
Third, if there wasn't a two-thirds majority in the Senate, what about annexation via joint resolution, as with Texas? Grant tried, but the votes were not there in the House:
"On June 29 debate was finally resumed in the Senate and on June 30 the treaty came to a vote. The result is reported as 28 to 28, or, counting pairs, 32 to 30 — a majority of but two, when a two-thirds vote was necessary. When one compares this with the attitude of the Senate on the St. Thomas treaty, it is clear that Grant's influence had accomplished a great deal.
"But this failure of the treaty did not cause Grant to desist. As, when foiled by Lee before Richmond, he moved by the left flank, so now he returned to the attack in the next ses-sion of Congress, by recommending in his annual message the annexation of San Domingo by resolution, "as in the case of Texas." J He also urged the appointment of a commission to negotiate a new treaty. The vote on the treaty in July had shown that possibly a majority of the Senate might favor a joint resolution, and the idea was not at once dispelled by the next step. Senator O. P. Morton, the leading supporter of the President, introduced a resolution for a commission of inquiry, which was considered on December 21 and 22. It was in this discussion that Sumner, shocked beyond measure at the naval support being furnished Baez, made his famous and bitter attack on Grant's policy as a "dance of blood," thereby stirring up an envenomed controversy. After a debate filled with stinging personalities, Sumner's amendments were rejected by large majorities and the resolution passed 32 to 9. On January 9 and 10 it was introduced into the House by Orth, who tried as on a previous occasion to prevent debate. He was overridden, however, and a general discussion of the whole subject took place, scarcely less bitter than that in the Senate. Charges of corruption were made and repelled. Swann asked :
"Why the interest which has been manifested in this whole subject ? Why do we see here day after day distinguished senators who are known to be in accord with the purpose which the President proposes to carry out ? Why do we see here the Secretary of the Navy and the Postmaster-General lobbying upon this floor for the purpose of passing through this extraordinary measure in such hot haste ? 8
"The result was different, for the House, on the motion of J. A. Ambler of Pennsylvania, adopted by a vote of 108 to 76 an amendment in these words : " provided that nothing in these resolutions will be held as committing Congress to the policy of annexing the territory of said republic." 4 This vote of the House killed the San Domingo scheme, for it was clear that annexation by joint resolution was not possible. The Senate adopted the House amendment, 1 and the commission was sent, consisting of Andrew D. White, S. G. Howe and B. F. Wade. This committee made a warmly favorable report, which Grant submitted to the Senate on April 5, 1871, but no action was ever taken."
http://archive.org/stream/jstor-2140255/2140255_djvu.txt
Incidentally, some of Grant's critics disputed that Texas had set a precedent for annexation of a new *territory* by joint resolution. All that the Texas precedent established, they argued, was that Congress, pursuant to its constitutional power to admit new states into the Union (Article IV, section 3) could admit *as a state* foreign territory (as well as territories already belonging to the United States). Since nobody proposed to admit Santo Domingo as a state (at least not immediately, though the treaty explicitly held out the possibility of later statehood) the only permissible method of annexation was by treaty. This argument would also be made--and this time rejected--in the case of Hawaii almost three decades later.
Just maybe if the whole process of the attempted annexation had been less sleazy, Sumner might not have been opposed and it might have gone through. But whether there was a non-sleazy way to get annexation, I do not know. And in any event, for some opponents (though not Sumner) the sleaziness argument was a cover for opposition to making more dark-skinned people US citizens...