Trump earlier called the attack a "horrific incident."
The June 15 campaign rally has been postponed.
"I don't want congrats, I want toughness & vigilance," Trump tweeted.
The attack left 50 people dead and many more injured.
A senior adviser to Sen. John McCain's (R-Ariz.) 2008 presidential campaign on Sunday criticized the way presumptive GOP presidential nominee Donald Trump is running his campaign, saying the candidate would have no data or field operation if not...
"Will she go as far as I would like her to go? No, she won't."
Sanders said he's a "great admirer" of Sen. Elizabeth Warren.
Sessions said Trump "probably" shouldn't have made the judge comments.
"Money is coming in. We're not worried about fundraising," said Paul Manafort.
Sanders said he wants to get a sense of what kind of platform Clinton will support.