Wyoming Senators Mike Enzi and John Barrasso both opposed the bill sponsored by Senate Democrats to extend tax cuts for all but the highest-earning Americans.
The two Republicans said the Democratic measure would hurt the ailing U.S. economy and actually would increase taxes on small business owners.
The bill passed Wednesday on a near party-line 51-48 vote in Washington.
Enzi and Barrasso supported a Republican alternative that would have extended the current tax provisions that have been in effect for about the last decade.
The Senate tax measure is not expected to go anywhere in the Republican-controlled U.S. House.