Jonathan Bernstein defends Republican Senators:

I see no reason to attribute conservative opposition to New START to anything other than conservative opposition to all treaties.

But Republicans don't oppose all treaties. From yesterday's NYT:

The down-to-the-wire suspense is unusual in the annals of arms control votes in the Senate. Most such treaties that reached the floor won by overwhelming margins if not unanimously. The rare arms control treaties to fail were generally never brought to a vote, with one exception being the Comprehensive Nuclear Test-Ban Treaty, which Mr. Kyl helped defeat in 1999.

Larison is worth reading on this question.