For both Rubio and Rand, running for president as well as reelection would pose challenges

But that’s not the only downside. Let’s suppose Rand Paul and Marco Rubio both run for president — and let’s suppose Rubio wins the nomination, while Paul drops out in time to qualify for re-election on the ballot.

It is entirely possible that this scenario would potentially cost Republicans two U.S. Senate seats (in what will already be a tough year for Republicans defending Senate seats).

At the very least, this could make both candidates look selfish. To avoid undermining the incumbent (or losing to him), Florida and Kentucky Republicans would presumably avoid fielding a candidate until the dénouement. This would put them at a severe disadvantage if a Democrat with statewide name recognition (think Rep. Debbie Wasserman Schultz or Alex Sink in Florida) seized the opportunity, and began aggressively running (and fundraising).