LAS VEGAS (AP) — Hillary Rodham Clinton and Sen. Bernie Sanders clashed on U.S. involvement in the Middle East, gun control and economic policy as they opened the first Democratic debate Tuesday, outlining competing visions for a party seeking to keep the White House for a third straight term.

Clinton was an aggressor from the start, an unexpected shift for a candidate who has barely mentioned her Democratic rivals since launching her campaign six months ago. Until now, Clinton and Sanders — who has emerged as her toughest competition — have circled each other cautiously and avoided personal attacks.

After Sanders, a self-described democratic socialist derided "a casino capitalist process by which so few have so much," Clinton said it would be a "big mistake" for the U.S. to turn its back on the system that built the American middle class. Asked whether she thought Sanders, who has a mixed record on gun control legislation, had been tough enough on the issue, she said simply," No, I do not."

Sanders defended his gun control record, and called for better mental health services, stricter background checks and closing a loophole that exempts gun shows from background checks

The two also tangled over foreign policy, an issue where Clinton is often more hawkish than others in the Democratic Party. The former secretary of state reiterated her call for more robust U.S. action to stop the Syrian civil war and defended her judgment on international issues, despite having voted for the 2002 invasion of Iraq.

Sanders called the Iraq war "the worst foreign policy blunder in the history of our country" and said he would not support sending American combat troops back to the Middle East to fight terrorism.

"Nobody does, Senator Sanders," Clinton interjected.

Joining Clinton and Sanders on stage in Las Vegas was a trio of low-polling candidates looking for a breakthrough moment: former Maryland Gov. Martin O'Malley; Jim Webb, a former Navy secretary and U.S. senator from Virginia, and Lincoln Chafee, the Republican-turned independent-turned Democrat from Rhode Island.

Not present but eager to make his presence known, Republican front-runner Donald Trump was commenting live on Twitter.

"Who knows, maybe a star will be born (unlikely)," Trump wrote shortly before the debate was to begin.

Also hanging over the debate: the lengthy deliberations of Vice President Joe Biden, who is weighing a late entry into the Democratic race. Debate host CNN kept an extra podium on standby just in case Biden decided to show up, but the vice president instead stayed in Washington, where he was watching the debate at his residence.

For Clinton, the debate was a much-needed opportunity to focus on policy in addition to the controversy over her exclusive use of personal email and a private Internet server during her tenure in the Obama administration. The email issue has shadowed her rollout of numerous policy positions and has hurt her standing with voters.

Clinton said her email use "wasn't the best choice" and cast the issue as a politically motivated effort by Republicans to drive down her poll numbers.

The former secretary of state has also faced criticism that she's shifted her positions on trade, gay marriage and other issues to match the mood of voters — a charge she vigorously denied Tuesday.

"Like most human beings, I do absorb new information, I do look at what's happening in the world," Clinton said. Pressed specifically on her newly announced opposition to a Pacific Rim trade deal she touted while serving in the Obama administration, Clinton said she had hoped to support it but ultimately decided it did not meet her standards.