(Adds Clinton, donor comments, details, background)
By James Oliphant and Amanda Becker
WASHINGTON, July 14 (Reuters) - U.S. Democratic presidential frontrunner Hillary Clinton welcomed the nuclear deal with Iran but emphasized that enforcement would be crucial, adding she had concerns about Tehran's "bad behavior."
"This is a very important moment," the former U.S. secretary of state said, speaking at the U.S. Capitol after meeting with House of Representatives Democrats.
"I think this is an important step that puts the lid on Iran's nuclear programs," Clinton said.
At the same time, Clinton emphasized that rigorous policing of the deal, including unfettered access to key Iranian program sites, would be critical to its success. She added that if she is elected president next year, she would be "absolutely devoted to ensuring that agreement is followed."
She also made clear that apart from the nuclear deal she would push to hold Iran accountable for state-sponsored terrorism and human-rights abuses. And she said she continued to view Iran as an "existential threat to Israel."
As the frontrunner for the 2016 Democratic presidential nomination, Clinton's careful remarks illustrated the delicacy of the issue for her.
As the former chief diplomat of the Obama administration, she has taken care to avoid diverging from the White House on foreign policy. At the same time, she has long spoken more forcefully than the president about confronting Tehran, and blunting that edge on national security could carry political risk.
When she was Obama's rival in 2008, Clinton was emphatic about retaliating against Iran should it ever launch an attack against Israel.
"I want the Iranians to know that if I'm the president, we will attack Iran," if it launches a strike against Israel, she said in a 2008 interview, adding that the United States "would be able to totally obliterate them."
Clinton also cannot risk alienating wealthy Jewish donors, some of whom may worry that the deal further endangers Israel.
David Golder, a Clinton donor in Chicago, praised her "pragmatic" response.
"I think if you look at engaged members of the Jewish community, they expect a good deal with Iran that doesn't put the U.S. or Israel at risk and defines for all of us what that means," he said. "There is a wide range of opinion in the Jewish community on the critical structure of these steps."
Clinton enjoys some maneuvering room, analysts said. Despite Republican efforts to vilify Obama's foreign policy, the president garnered about 70 percent of the Jewish vote in 2012, suggesting that many American Jews are far from being single-issue voters primarily focused on Israel's security. (Additional reporting by Emily Flitter in New York; Editing by Jeffrey Benkoe)