This Politico story started the speculation (which seems completely unjustified), and it features a law professor saying this wouldn’t be a bribe.
I think the question is whether it is illegal to pay a candidate to withdraw from a race. Such conduct is definitely illegal in some states, but it is not clear to me that such laws would apply in a federal election.
Dan Lowenstein’s seminal article on bribery in UCLA discusses a state law in California making this illegal (A person shall not . . . pay, solicit, or receive . . . any money or other valuable consideration . . . in order to induce a person not to become or to withdraw as a candidate for public office)
I cannot find an analogous federal law, and did find this in a federal election crimes manual from 2007:
(b) Promise of appointment by candidate: 18 U.S.C. § 599 This statute prohibits a candidate for federal office from promising appointments “to any public or private position or employment” in return for “support in his candidacy.” It is one of the few federal criminal laws specifically addressing campaign-related activity by candidates. It is a class statute that applies only to misconduct by federal candidates. Willful violations are two-year felonies; nonwillful violations are misdemeanors. Section 599 has potential application when one candidate attempts to secure an opponent’s withdrawal, or to elicit the opponent’s endorsement, by offering the opponent a public or private job. See also 18 U.S.C. § 600, discussed above. It also applies to offers of jobs by federal candidates to others to secure endorsements. While Section 599 does not reach offers or payments of money to secure withdrawal or endorsements, if the payment was not reported accurately, such matters may be prosecutable as a reporting violation of FECA under 2 U.S.C. §§ 434(b) and 437g(d) (my emphasis)
Any other potentially applicable law?