A Rubio adviser stressed nothing has been nailed down for any kind of announcement, but the timing makes sense: Likely presidential candidate Rand Paul is expected to make things official April 7, followed by a five-day, five-state announcement tour, so Rubio presumably would not want to share the spotlight during that period.

All-but-announced candidate Jeb Bush appears to be in no rush to shift more formally into campaign mode, but Texas Sen. Ted Cruz made his announcement this week, and Democrat Hillary Clinton is expected to make her campaign announcement in April as well, though nothing has been set.