"They can try to hijack another party and get on the ballot, but, look, it's a suicide mission for our country because what it means is that you're throwing down not just eight years of the White House but potentially 100 years on the Supreme Court and wrecking this country for many generations," Priebus said on "Fox News Sunday."
"So I think that's the legacy these folks will leave behind. I think it's very dangerous, and there's other ways to get assurances on the things that they're worried about."
A group of veteran Republicans, including 2012 presidential nominee Mitt Romney, are taking steps that could produce a third-party bid to challenge Trump in November, The Washington Post reported Saturday.
In addition to commissioning opinion polls and lining up funding, figures such as Romney have reached out to Sen. Ben Sasse (R-Neb.) and Trump's former rival Ohio Gov. John Kasich (R) as potential recruits.
Priebus called the approach taken by House Speaker Paul D. Ryan (R-Wis.), who met with Trump on Thursday, "the better way to go," adding that he would be surprised if Ryan did not endorse the presumptive GOP nominee soon.
The party chairman also made a dig at Romney, saying he is "obsessing" unnecessarily over Trump's refusal to release his tax returns.
"People don't look at Donald Trump as to whether he releases his taxes," Priebus said on ABC's "This Week." "People look at Donald Trump and say, 'Is this person going to cause an earthquake in Washington, D.C., and make something happen?' That is it. That is how he is being judged by the American people. So all these things that we've been analyzing for a year, and that Mitt Romney is obsessing over — it hasn't done a thing."