Hillary Clinton aims to corral the final delegates needed to clinch the Democratic U.S. presidential nomination after a long struggle against Bernie Sanders on Tuesday, with voters casting ballots in six states, as she ramps up attacks on presumptive Republican nominee Donald Trump.

Voters in California, Montana, North Dakota, South Dakota, New Jersey and New Mexico will vote in nominating contests on Tuesday. Clinton has scheduled rallies in the Los Angeles area on Monday.

The Clinton campaign hopes the former secretary of state will effectively lock up the nomination early on Tuesday evening with a victory in New Jersey, taking the focus off the fractious battle she has been fighting in California against Sanders. Clinton may declare herself the party's nominee before the polls close there.

In California, the biggest electoral prize, Clinton once held a sizable lead over Sanders, a U.S. senator from Vermont, but polls in recent days show a dead heat between the two.

A loss to Sanders in California could galvanize Sanders and his supporters to carry their insurgency campaign all the way to the party's nominating convention in July. A loss could also reinforce the argument of Trump that Clinton is a weakened candidate.

Clinton has 2,357 delegates going into Tuesday's contests, just 26 short of the 2,383 she needs to clinch the nomination at the convention in Philadelphia next month. Trump already has secured the delegates he needed to clinch the Republican nomination for the Nov. 8 presidential election.

TRUMP ON THE DEFENSIVE

Trump, however, was on the defensive over comments about Mexican-American U.S. District Judge Gonzalo Curiel, who is overseeing fraud lawsuits against Trump University, the New York businessman's defunct real estate school.

Trump has suggested that Curiel's Mexican heritage was influencing the judge's opinion about the case because Trump has campaigned on a platform of building a wall along the U.S. border with Mexico to stop illegal immigration.

"All I want to do is figure out why I'm being treated unfairly by a judge. And a lot of people agree with it," Trump said on Monday on the Fox News program "Fox & Friends."

Clinton, who has denounced Trump's comments about Curiel, has won the support of minority voters in her race against Sanders and hoped for a strong showing in California, the state with the largest Latino population.

Last week, the state reported that the tally of registered voters in California was the highest in history at 18 million, with more than 650,000 registering in the past six weeks. Three out of every four new voters was a Democrat.

In a speech in San Diego last week, Clinton attacked Trump on his national security credentials, arguing he lacked the temperament and knowledge to engage with other nations.

Clinton said Trump would be dangerous if left in charge of the nation's nuclear arsenal, arguing he could use them if offended because he has "thin skin."

The speech was praised by Democrats who said it could bolster Clinton's efforts to show she could effectively take on Trump in the general election. Even some Republicans said the speech hit a major vulnerability for Trump.

Trump, appearing on CNN, responded to Clinton’s attack.

"I don't have thin skin. I have very strong, very thick skin," Trump said. “I have a strong temperament, and it's a very good temperament, and it's a very in-control temperament, or I wouldn't have built this unbelievable company.”

The latest Reuters/Ipsos tracking poll shows Clinton with an 11-percentage-point edge over Trump, 46 percent to 35 percent, a marked change from just 10 days ago, when fewer than 4 percentage points separated the two.