WASHINGTON (AP) — Election Day just 15 days off, Donald Trump is fighting to preserve his narrow path to the presidency in must-win Florida on Monday as Hillary Clinton tries to slam the door on her Republican opponent in battleground New Hampshire.

At the same time, Democrats continue to get help from President Barack Obama, whose high job approval numbers have made him a political force in the sprint to Nov. 8. The president lashed out at Trump and praised Clinton as he campaigned Sunday in Nevada, a competitive state in the race for the White House and the Senate.

Obama told Nevadans they have a winning hand in Clinton and Senate candidate Catherine Cortez Masto, who is locked in a tight race to fill the seat being vacated by retiring Democratic Sen. Harry Reid. Democrats need to retain the Nevada Senate contest and pick up four new seats elsewhere to claim the Senate majority if Clinton wins.

Many Republicans fear that Trump's struggles could drag down his party's chances in competitive House and Senate elections across the nation.

The president was unsparing Sunday in his criticism of Trump, describing the billionaire businessman as unfit to be president. Obama also railed against Republicans and conservative media outlets for promoting "all kinds of crazy stuff" about him and his party's leaders. He cited as an example the years-long questions from Trump and others about whether he was born in the U.S.

"Is it any wonder that they ended up nominating somebody like Donald Trump?" Obama said.

Trump, meanwhile, lashed out at his Democratic opponent on Twitter on Monday, claiming that "Crooked Hillary" wants the United States to accept "as many Syrians as possible" from the war-torn region. Clinton has said Obama's plan to admit 10,000 Syrian refugees this fiscal year is a good start, but that the nation "needs to do more."

Trump was campaigning Monday in Florida, a state his advisers concede he must win to have any chance at becoming president.

The spotlight on Florida shined brighter on Monday also because in-person early voting was beginning across 50 counties, including the state's largest: Broward, Duval, Hillsborough, Miami-Dade, Orange and Palm Beach. The remaining counties will start in the coming week.

Early voting by mail has been underway for weeks. Nearly 1.2 million voters in Florida have already mailed in ballots. The state has nearly 13 million registered voters.

Clinton's running mate, Virginia Gov. Tim Kaine, was set to make two campaign stops in Florida on Monday. Clinton plans to visit the state Tuesday.

Clinton's focus on Monday was New Hampshire, a state that offers just 4 electoral votes compared to Florida's 29, but marks a key piece to Trump's increasingly narrow path to the 270 electoral votes needed to win the presidency.

The Trump campaign acknowledged its challenge in a Monday fundraising email, conceding that victories in tossup states like Florida, Ohio, Iowa, Nevada and North Carolina wouldn't be enough to reach 270.

"Polls show us close in New Hampshire, Colorado, and Pennsylvania. Winning just any one of those states would lead us to victory," the campaign wrote.