Ed Kilgore thinks they will eventually take aim at Medicaid:

Medicare beneficiaries are the very core of the GOP's political base at present; Medicaid beneficiaries decidedly are not. Moreover, as I argued last year, for all the pundit hilarity about people receiving socialized health insurance via Medicare railing against socialized health insurance, many of these folk think of their coverage as an earned benefit, not as any form of government largesse. So there's nothing inherently implausible politically about the GOP just flatly defending Medicare (and for that matter, Social Security) while going after the lazy welfare bums under the age of 65. Some of you may have read Tom Edsall's recent dark vision of an impending era of scarcity wherein politics is dominated by generational and class battles over who gets what from government. Thanks to the central position of older white voters in the GOP, and of Medicare in the federal budget, this nasty scenario could arrive a lot faster than even Edsall has imagined.