(AP) -- Democrat Hillary Clinton says Republican Donald Trump has become recruiting fodder for Islamic extremists, and Trump calls her a liar, in what could be a preview of the 2016 general election campaign for president.

Trump said Monday that Clinton lied when she said during a Democratic Party debate that the Islamic State group used videos of his comments about Muslims to recruit militants.

The Republican presidential front-runner also took to Twitter to demand an apology from his leading Democratic opponent.

"She's a liar!" Trump told more than 6,000 people at a rally in Grand Rapids, Michigan on Monday evening. He also said Clinton was "crooked" and "not a president."

During Saturday's faceoff among Democrats, Clinton said Trump had become the Islamic State's "best recruiter" and said, "They are going to people showing videos of Donald Trump insulting Islam and Muslims in order to recruit more radical jihadists."

There was no evidence to back the claim, and a spokeswoman later said, "She didn't have a particular video in mind."

Still, Clinton campaign spokesman Brian Fallon, asked Monday by MSNBC if Clinton would apologize to Trump, insisted, "Hell no."

During his rally Monday at Grand Rapids' DeltaPlex Arena, Trump used crude language to portray Clinton as someone who constantly loses, citing her race against Barack Obama for the Democratic nomination in 2008.

"She was favored to win and she got schlonged, she lost," he said.

He also mocked Clinton for returning to the debate late following a commercial break. "I thought she quit, I thought she gave up," Trump joked.

He later seemed to acknowledge reports that she got delayed waiting to use a restroom: "I know where she went. It's disgusting, I don't want to talk about it."

Trump argued that he is the last person Clinton wants to run against in a general election.

"Ask Jeb Bush if he enjoys running against me," he said of the former Florida governor who has been struggling to gain traction despite a massive early fundraising advantage.

"Ask Lindsey Graham, did he enjoy running against Trump?" he said of the South Carolina senator who on Monday announced his departure from the race.

Trump also defended the kind words he's been exchanging with Russian President Vladimir Putin, brushing off criticism that he has been too kind to the Russian president.

"That's like a good thing, not a bad thing," he insisted. "Wouldn't it be nice if we could get along like with people?"

And he made clear that he is opposed to the killing of journalists, after appearing to brush off concerns about Putin's record on a Sunday morning news show.

"I don't like that, I'm totally against that," said Trump. "By the way, I hate some of these people ... and some of them are such lying, disgusting people, it's true. But I would never kill. And anybody that does I think would be despicable."