(Sen. Jeff Flake of Arizona.AP Photo/Jacquelyn Martin)

Donald Trump lashed out at Sen. Jeff Flake on Sunday after the Republican from Arizona said he wasn't ready to vote for Trump.

"The Republican Party needs strong and committed leaders, not weak people such as @JeffFlake, if it is going to stop illegal immigration," Trump said on Twitter Sunday night.

He then tweeted an hour later: "The Great State of Arizona, where I just had a massive rally (amazing people), has a very weak and ineffective Senator, Jeff Flake. Sad!"

In an interview with Jake Tapper on CNN's "State of the Union," Flake, a vocal Trump detractor, said he "simply can't" support Trump.

"I would not vote for Hillary Clinton, and as of now, I would still not vote for Donald Trump," Flake said.

"I just know that I would like to vote for Donald Trump. It's not comfortable to not support your nominee," he added. "But given the positions he's taken, and the tone and tenor of his campaign, I simply can't."

Flake also suggested that Trump is in danger of losing Arizona to Democratic candidate Hillary Clinton, a belief he's held publicly since June. Arizona has voted Republican in every election except one since 1952. Bill Clinton in 1996 was the only Democrat to carry the state in that span.

(Donald Trump campaigning in Arizona.REUTERS/Mario Anzuoni)

"Arizona should still be a red state. But Donald Trump, with the rhetoric that he's under and the characterizations of, you know, many of the state's population, have put the state in play," Flake said.

Arizona is more than 30 percent Hispanic, according to the latest Census data.

Independent presidential candidate Evan McMullin came to Sen. Flake's defense with a jab at Trump, saying the GOP nominee is "[demonstrating] his fragility" by attacking people who oppose him.

Clinton joined the fray, adding, "There's going to be no one left in Washington for Trump to work with who he hasn't insulted."

The real-estate mogul has been criticized from both sides of the aisle after doubling down on his hard-line immigration platform, which includes building a wall along the US-Mexico border.

View Trump's tweets below:

Watch Flake's interview on CNN here:

Jeff Flake stands his ground: "I simply can't" support Donald Trump. https://t.co/4CGZCNq2UM