If consent to the application of the provisions of section 2513 is signified as provided in § 25.2513-2 , and not revoked as provided in § 25.2513-3 , the liability with respect to the entire gift tax of each spouse for such "calendar period" (as defined in § 25.2502-1(c)(1) ) is joint and several. See paragraph (d) of § 25.2511-1 .
26 C.F.R. §25.2513-4