Who is preferred JD or Marco?

The most striking thing about this race is the velocity of the shift. Back in February, Vance had 52% and Rubio had 20% in the Emerson College national poll. By May those same likely Republican primary voters had them at 36% and 35%, a single point apart. That is not a drift, that is a realignment.

In a full field with other names included, Vance technically leads, but when you strip the race down to just the two of them, the picture flips. An Overton Insights survey put it directly: in a binary ballot test, Rubio leads Vance 42% to 36%.

Where each of them is winning

The age split is the most telling structural detail. Rubio leads among Republican primary voters over 50 by a six-point margin, 41% to 35%, while Vance leads the under-50 vote, 37% to 26%. Older voters tend to show up more reliably in primaries, which is a structural edge for Rubio. But as the Emerson director noted, if you are Vance, you have some concern about hanging your hat on the youth vote, while if you are Rubio, you are looking at that older vote and you are winning it.

The one variable that overrides all of this

Political observers note that Donald Trump's endorsement could be the deciding factor. Trump is still the decider in the Republican Party, and whoever he endorses as his successor will not just be the leader, but the likely nominee. Both men know this, which is probably why Vance has publicly called Rubio his closest friend in the administration. They are competing while pretending not to, which is itself a kind of political skill.

If Trump stays neutral, this is genuinely open. If he picks one, the other is probably done.