The most retweeted tweet during the first day of the Republican National Convention was posted by the GOP's chief opponent.

"Republicans have a lot of nice things to say - when it's not an election year. Thanks, Mayor Giuliani. #RNCinCLE," presumptive Democratic presidential nominee Hillary Clinton tweeted Monday night.

In the tweet, Clinton quoted former New York City Mayor Rudy Giuliani, who said of Clinton in 2010: "I think she is doing a good job. ... I have tremendous respect for her."

The Republican National Convention kicked off Monday.

Giuliani spoke shortly before Clinton's tweet, as part of the GOP's theme of "Make America Safe."

In his speech, the former New York City mayor tore into Clinton's "vicious, nasty campaign."