"St. Jacqueline the martyr" would be a very potent image which the Kennedy team could exploit for the 1964 elections. The American people will grieve with him and his children as much, perhaps more than they did with Jackie and the children. If a president is murdered, well that is a hazard of the job, and it had happened before. For a First Lady to be murdered like that, unspeakable in 1963.
Her murder, and the image of the widowed father soldiering on in raising the children while trying to lead the nation would also be a powerful image which could make the President very hard to assail. In the immediate aftermath, anyone saying anything critical will have to beware of sounding mean-spirited. While this will fade over time, it will remain a central part of the narrative for the rest of his presidency, and the Kennedy brothers had access to expert p.r. people who could make that work.
Rouge Beaver said:
As a Senator, he couldn't pass bills of his own (but passed many important amendments) because he was among the 15 most junior Senators during his tenure
One of the things that irked RFK most during his Senate career was that in the Senate he was
junior to his younger brother Teddy. That was accentuated in 1967 when Ted became the senior Senator from Massachusetts while Robert was still the junior Senator from New York. Ted had prefferment in the Senate's seniority system.
RFK wasn't ready for a Presidential run in 1964; for one thing he was only 39 years old and he didn't have a political resume outside the shadow of his brother. In some ways Teddy's career progressed along the Presidential route much faster because being in the Senate allowed him a degree of freedom from that JFK shadow and to make his own mark.
While being Sec Def. will help RFK I would think, for his long term political development, it would be best to send him off to be elected Governor of Massachusetts where he could develop a political and executive record of his own, independent of JFK's.
The DPOTUS thing doesn't work out very well at that time because there is no historical context for it as part of a public career, and flirts dangerously with the idea of an absentee president, or one who can't cut it. As it is the closest thing to a DPOTUS is usually the President's Chief of Staff, and that isn't usually a stepping stone to the presidency.