"I've got to think, directly or indirectly, it's from the Clinton campaign," he said.
Clinton and Trump are tied at 38 percent support in the swing state of Iowa.
OPINION | Holt must push answers on energy and climate change tonight
Clinton aide says Trump hoping to "coast on style and his television showmanship."
OPINION | Holt needs to ask tough questions on lies, immigration and gender issues.
Rafael Bernal 09/26/16 02:10 PM EDT
Monday's U.S. presidential debate could further rattle Mexico's economy.
The Libertarian nominee will hold a Facebook Live event.
Kaine also criticized Trump for his past comments about John McCain.
The conservative Iowa lawmaker's tweet came just hours before the first debate.
A raft of new polling data shows the race is a toss-up.