Donald Trump lashed out at conservative political analyst Bill Kristol on Twitter Sunday after Kristol hinted the businessman could face competition from an independent candidate in November.

"Just a heads up over this holiday weekend," Kristol said on Twitter. "There will be an independent candidate--an impressive one, with a strong team and a real chance."

The presumptive Republican nominee wasted no time firing back at Kristol:

Trump locked up the Republican nomination earlier this month, but questions linger about his ability to unify the party.

Kristol, editor of the conservative opinion magazine The Weekly Standard, has openly speculated for weeks about enlisting an independent candidate to take on Trump in the general election.

Possible names that have been floated include 2012 Republican candidate Mitt Romney, Ohio Gov. John Kasich, Nebraska Sen. Ben Sasse and even billionaire businessman Mark Cuban.

Kristol was one of 22 prominent conservative thinkers who signed an anti-Trump manifesto in January, when Trump was narrowly leading a wide-open Republican primary with 12 candidates.