2. Jon Kyl
For months, Senator Kyl (R-Ariz.) has been the GOP's de facto leader on a host of foreign-policy issues, not least of which is the ratification of the New START nuclear reductions treaty with Russia. Kyl is withholding his support of New START until he gets a litany of concessions from the White House, and he might not even support the treaty at all. If the GOP makes significant gains in the election, Kyl will have a strengthened argument for pushing the full Senate vote on the treaty to next year: he could very well argue that the incoming GOP senators have a right to vote on the treaty, complicating further the administration's drive to secure the 67 votes needed for ratification.
An increase in the number of Republican senators will also provide Kyl with more leverage to bargain for all sorts of things, such as more money for nuclear modernization, before he releases other members of his caucus to vote in favor of the treaty. Kyl has also been involved in the ongoing GOP effort to hold up the confirmation of several nominees for ambassadorships, such as Robert Ford and Frank Ricciardone. Increased GOP numbers could force the administration to take more seriously Kyl's demands for more access to State Department communications and more explicit statements on the administration's foreign-policy positions if it wants to see these ambassadors confirmed.