Romney, who has not endorsed any of the presidential hopefuls, will join Kasich at campaign events in North Canton and Westerville, Ohio, on Monday, according to the Kasich campaign.
The 2012 Republican nominee is making a campaign trail comeback as part of his escalating efforts to prevent Donald Trump from clinching the Republican nomination.
Romney laid out the case against Trump in a high-profile speech earlier this month, in which he called him a "phony" and a "fraud." Since then Romney has offered to pitch in to help the remaining GOP candidates -- Kasich, Marco Rubio and Ted Cruz -- defeat the billionaire businessman.
"He's been clear that he believes that Donald Trump is not the best person to represent the Republican Party and will do what he can to support a strong nominee who holds conservative values to win back the White House," said a source close to Romney. "Governor Romney looks forward to joining Governor Kasich on the trail tomorrow in Ohio."
The Romney assist in Ohio comes as Trump has sharpened his jabs against Kasich on the campaign trail, labeling him an "absentee governor."
"We are excited to have Governor Romney join us in Ohio tomorrow," said Chris Schrimpf, a spokesman for the Kasich campaign. "We believe that Governor Kasich is the best candidate to defeat Donald Trump in Ohio and to defeat Hillary Clinton in November."
The latest polls show Trump trailing Kasich by five to six percentage points in Ohio. In the latest indication that he now sees the Ohio governor as an emerging threat, Trump revamped his campaign schedule on Monday to add an event in Ohio.
On the campaign trail in Boca Raton, Florida, Sunday evening, Trump touted his polls in the Sunshine State but acknowledged, "In Ohio, it's pretty even with Kasich."
Earlier this month, Romney recorded campaign calls for Rubio, as well as Kasich, in an attempt to help boost their turnout in states like Michigan.
At the time, Kasich accepted Romney's help -- but only begrudgingly.
"We want to make sure people don't think that Romney's for somebody else and not for me," Kasich said at the time. "So is this the scenario that I would have liked, to have it work out like this? Not really."
The closer embrace is a strategic move for the Kasich campaign. While Romney narrowly beat Rick Santorum in the Ohio primary in 2012, his win was bolstered by key counties where Kasich will need to run up the score to beat Trump.
Romney will join Kasich in two of those areas -- North Canton, which sits in Stark County, and Westerville, which straddles Delaware and Franklin counties -- on Monday.
Late Sunday night the Kasich campaign sought to capitalize on Romney's campaign stops with a fundraising pitch.
"We just announced that former Massachusetts Governor and 2012 Republican Nominee Mitt Romney will join Gov. John Kasich on the campaign trail in Ohio on Monday," read one email solicitation. "The eyes of the nation are on Ohio and the latest polls show Gov. Kasich defeating Trump in Ohio. We're on the verge of victory but need to hit our final fundraising goal to get there!"