If annexation fails in 1898, it will succeed in 1899. This is also true of the Philippines. Observe the result of the 1898 election:
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55th Congress (1897-1899)
Majority Party: Republican (44 seats)
Minority Party: Democrat (34 seats)
Other Parties: 5 Populist; 5 Silver Republican; 2 Silver
Total Seats: 90
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56th Congress (1899-1901)
Majority Party: Republican (53 seats)
Minority Party: Democrat (26 seats)
Other Parties: 5 Populist; 3 Silver Republican; 2 Silver
Vacant: 1
Total Seats: 90
https://www.senate.gov/pagelayout/history/one_item_and_teasers/partydiv.htm