Ain't gonna work.
Romney may call Trump disingenuous now, but it only makes Romney look disingenuous. Four years ago, Romney relished the limelight that came with a Trump endorsement.
Romney's a guy who distanced himself from Reaganesque policy in making an unsuccessful run for the U.S. Senate in 1996. He's also a guy who self-labeled as progressive when running for Massachusetts governor in '02.
In 2012 it "meant a great deal" to Romney to get endorsement of @realDonaldTrump, described him "as a more successful businessman than I."— Laura Ingraham (@IngrahamAngle) March 3, 2016
Romney's got zero credibility to attack Trump.
The two are more alike than different.
Update: Looks like someone yanked the video where Romney called himself a progressive. Here's alternative link to the same clip from another YouTube source.