ANNANDALE, Va. — Senator Tim Kaine had already mastered the requisite body language of a vice-presidential hopeful.

He nodded vigorously. He pumped both fists. He proved a loud and demonstrative clapper.

“Are we ready for Hillary?” he asked a crowd of more than 1,000 at a community college gymnasium here on Thursday.

The people roared.

Less clear: Is Hillary Clinton ready for Mr. Kaine?

With Mr. Kaine, a first-term senator from Virginia, moving to establish himself as a leading Democratic vice-presidential contender, the event doubled as his most conspicuous audition yet for the role. During a 15-minute introduction of Mrs. Clinton, Mr. Kaine seemed inclined to capitalize.

Test-driving catchphrases, talking with his hands, slipping effortlessly into Spanish, Mr. Kaine praised the presidential candidate beside him as she grinned and nodded, clasping her hands at her midsection.