Senators Marco Rubio of Florida and Rand Paul of Kentucky are both seriously considering running for the Presidency, but they both face the quandary that if they so decide, they must forfeit running for reelection to the Senate in 2016. State laws in Florida and Kentucky prevent them running for both jobs at the same time.
This was not the case in other Presidential elections, as shown by:
Lyndon B. Johnson running for Vice President in 1960 while running for reelection to the Senate in Texas.
Lloyd Bentsen running for Vice President in 1988 while running for reelection to the Senate in Texas.
Joe Lieberman running for Vice President in 2000 while running for reelection to the Senate in Connecticut.
Paul Ryan running for Vice President in 2012 while running for reelection to the House Of Representatives in Wisconsin.
All four were elected to their Congressional seats, with only Johnson vacating it to become Vice President.
But notice that in all four of these cases, the position they were running for was Vice President, not President.
So both Rubio and Paul face a difficult situation, and one would advise that they think long and hard about giving up their Senate seats, even though both seem, clearly, too ambitious to be President, and do not seem to love their Senate positions, as simply too restrictive for their egos!