WEST PALM BEACH, Florida (Reuters) - President Barack Obama cited the "ruthless" suicide bus attack in Bulgaria and upheaval in Syria as reasons to fortify support for Israel, telling a re-election campaign event in Florida on Thursday he stood by Israel's side.

Departing from his normal stump-speech themes of tax fairness, education and healthcare, the Democratic president described the Bulgaria airport bus bombing that killed Israeli tourists as "barbaric" and a "ruthless terrorist attack."

"I want everybody here to know under my administration, we haven't just preserved the unbreakable bond with Israel, we have strengthened it," Obama told a mainly older audience in Florida, an electoral battleground state with many Jewish voters.

"This is a moment of great uncertainty in the Middle East, given what's happening in Syria and given what's happening in other places. Now is the time to make sure that we do everything we can to protect Israel's security," he said.

Israel has accused Iran of being behind the Bulgaria attack. Obama did not mention Tehran in his remarks or specify how the United States would proceed to boost support further for Israel.

(Reporting By Laura MacInnis; Editing by Peter Cooney)