CLEVELAND — Jeb Bush has been practicing all summer for this moment: his first big chance to convince voters he is a reliable problem-solver fit to sit in the Oval Office.

Scott Walker has been boning up on issues foreign and domestic, eager to prove that after a rocky debut this spring he has amassed the knowledge to be a credible commander in chief.

Ted Cruz spent this week ensconced with advisers inside a Capitol Hill townhouse honing the rhetorical and theatrical skills that made him a standout on the college debate circuit.