(CNN) - Rudy Giuliani hailed former Florida House Speaker Marco Rubio Monday as the "one candidate that [Florida Republicans] can trust" to uphold conservative values in Washington.
"This is an easy choice in this race," the former New York City mayor said at a press conference in Miami before a gathering of Rubio supporters. "This is an easy choice for a Republican because he is the one candidate that we can trust to represent Republican principles in Washington and remain a Republican if he goes to Washington."
Rubio will face Florida Gov. Charlie Crist in the GOP senatorial primary in late August.
Giuliani's endorsement is among Rubio's most high profile to date, and is being seen as political payback to Crist who refused to endorse the former New York City mayor during his presidential bid two years ago.
Crist instead lent his last-minute support to Arizona Republican John McCain, who ultimately won that state's pivotal presidential primary and effectively ended Giuliani's hopes of becoming the GOP's presidential nominee.
A Mason-Dixon poll late last month showed Rubio leading Crist among Republican voters by 11 points, 48-37 percent. Fifteen percent in that survey remained undecided.
But it was almost as if Giuliani was back on the presidential campaign trail himself Monday, using much of his 10-minute endorsement speech to criticize President Obama's policies - specifically his approach to the war on terrorism.
"Let's have an America that stands up to terrorism, not an America that has ambiguity and confusion," Giuliani said. "Do we want to see people like Khalid Sheikh Mohammed tried in New York? Or Miami? Shouldn't he be tried in a military tribunal?"
"The president can't make up his mind," he continued. "Do you understand what kind of lack of leadership that shows to the rest of the world? I tried over 4,000 criminals. The easiest decision I had was where to try them."