President Obama reflected tonight at a campaign rally in Las Vegas on what he said has been a "tough day" for the country, mourning the loss of four Americans killed in Libya, vowing justice for their killers and pledging that U.S. diplomacy would be unwavering "because the world needs us."

Obama, who opted to push ahead with his battleground-state swing less than 24 hours after the deadly attack, used a somber tone to address his boisterous supporters, waving off cheers from members of the crowd at the top of his remarks.

Today is "a reminder that the freedoms we enjoy - sometimes even the freedoms we take for granted - they're only sustained because there are people like those who were killed, who were willing to stand up for those freedoms, who were willing to fight for those freedoms, in some cases to lay down their lives for those freedoms," Obama told the crowd. "Tonight, let's think of them."

The president echoed a goal he stated earlier in the day to bring the killers to justice and to persist in spreading American democratic values. He said the United States remains an "indispensable power" around the globe.

"We will not be deterred. We will keep going. We will keep going because the world needs us. We are the one indispensable power in the world," Obama said. "And if we are going to see peace and security for our children and grandchildren, then that means this generation of Americans has to lead. We're going to have to keep doing the work no matter how hard it seems sometimes."

The president then pivoted to election-year politics, delivering his argument for the country's economic future while highlighting contrasts with the GOP presidential ticket.

In the 30-minute speech, however, Obama never once mentioned by name his Republican rival, Mitt Romney, or Romney's running mate, Rep. Paul Ryan, R-Wis. He only referred to his challengers as "the other side" and his "opponents."

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