A Look Inside Four 'Stop Trump' Efforts Against the GOP Front-Runner

— -- There are several groups trying to stop Donald Trump, but what are they actively doing to slow down the Trump Train? From Mitt Romney’s voice to a secret confab and a ramp up of advertising, here are the ways different GOP groups are trying to “Stop Trump.”

1. Romney Robocalls

Mitt Romney hasn’t endorsed one of Donald Trump’s GOP rivals, but he has recorded robocalls for at least two of them. The robocalls encourage voters to cast ballots for John Kasich and Marco Rubio, telling them it is important to elect "a candidate who can defeat Hillary Clinton and who can make us proud." The robocalls started going out to voters in Michigan this morning, Kasich spokesman Rob Nichols told ABC News.

"Hello, this is Mitt Romney calling, and I'm calling on behalf of Kasich for America," the recording begins, referring to the Ohio governor's campaign. "Today, you have the opportunity in Michigan to vote for a Republican nominee for president. These are critical times that demand a serious, thoughtful commander in chief."

In a statement this morning, a Romney aide said, “Governor Romney has offered and is glad to help Senator Marco Rubio, Senator Ted Cruz, and Governor John Kasich in any way he can. 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.”

The Kasich calls are running in Michigan and the Rubio calls are running in Michigan, Idaho and Hawaii. Yes, you read that right. The robocalls for both Kasich and Rubio are both running in Michigan. There are currently no robocalls in any states helping Ted Cruz, but today he was asked about Romney helping Rubio and Kasich. He seemed open to the idea of Romney helping him too, admitting that he’s exchanged calls and emails with 2012 GOP nominee.

"We’ve been in conversations with Gov. Romney. He’s very graciously reached out to our campaign and I’ve talked with him on the phone. We’ve been trading emails with him and I’m grateful for the support we’re receiving from leaders all across the country,” Cruz said.

2. Spending

Donald Trump’s campaign is being outspent by at least more than 2-1 by anti-Trump groups in Florida (see chart below). Total spending by anti-Trump groups so far totals at least $13 million -- just from American Future Fund, Our Principles PAC and Club for Growth Action. This does not include likely anti-Trump spending from Marco Rubio’s super PAC, Conservative Solutions PAC, because although they spent money on anti-Trump advertising, they are also running pro-Rubio ads and there is no way to separate that spending.

In Florida, the Trump campaign has spent $1,988,920 compared to the $1,026,193 by Our Principles PAC, the $1,983,512 by the American Future Fund, the $1,046,541 by the Club for Growth, all of which are actively opposing the billionaire real estate developer.

3. AEI Sea Island Confab

GOP leaders, billionaires and tech gurus came together last weekend at the conservative American Enterprise Institute’s annual secretive World Forum in Sea Island, Georgia. It’s a private resort and the yearly confab is always off the record, but a guest told ABC News that Donald Trump and how to stop him was the talk of the halls at the forum.

Attendees included Karl Rove, as well as GOP senators, including Tom Cotton of Arkansas, Ben Sasse of Nebraska (he has already said he will not vote for Trump), Cory Gardner of Colorado, and Tim Scott of Georgia, as well as GOP donors, and tech CEOs, including Apple CEO Tim Cook and Tesla Motors head Elon Musk.

Weekly Standard founder Bill Kristol attended and described in an emailed newsletter about the conference that the stop Trump talk was a “specter haunting the World Forum -- the specter of Donald Trump,” borrowing lines from the Communist Manifesto. “There was much unhappiness about his emergence, a good deal of talk, some of it insightful and thoughtful, about why he's done so well, and many expressions of hope that he would be defeated,” Kristol said of the Trump talk at the confab.

“The attendees at the forum were businessmen and scholars, mostly, so the expressions of hope, and injunctions to do something were directed at those who were thought to have some ability to influence events -- such as magazine editors. I of course tried to explain the limits -- shocking but true -- to the power of magazine editors. But I did come away with this conviction reinforced: There's obviously lots of analysis of Trump and Trumpism to be done, and in fact we've done a fair amount of that in the pages of The Weekly Standard. But the key task now, to once again paraphrase Karl Marx, is less to understand Trump than to stop him. In general, there's a little too much hand-wringing, brow-furrowing, and fatalism out there and not quite enough resolving to save the party from nominating or the country electing someone who simply shouldn't be president.”

The same attendee source said many of the GOP donors were “uncertain about how to stop him, with many worried Trump is already inevitable” and any efforts to stop him could “backfire.” The same source noted that Cook, Musk and the tech gurus were not in on the stop Trump chatter.

4. Ads

There are now a substantial amount of anti-Trump television and digital ads running. They are from third-party groups like Our Principles PAC, the anti-Trump PAC, as well as the anti-tax group, Club for Growth, Rubio’s super PAC, Conservative Solutions PAC, the conservative PAC American Future Fund, and even an anti-Iran Deal group Veterans Against the Deal.

There were no new ads today, but our compilation of anti-Trump advertising shows American Future Fund Political Action, which they say is separate from American Future Fund, is out with one TV ad in Florida starting Monday.

American Future Fund has run at least six TV ads, most recently releasing two this weekend as part of a $2 million buy. The Club for Growth is running at least three ads, part of four buys, including most recently a $2 million buy in Illinois launched Monday. Rubio’s super PAC has run at least three anti-Trump ads. OPP has run at least one TV ad and one digital ad. And finally Veterans Against the Deal released both a TV ad running in Florida and Ohio and a digital ad.

ABC News’ Ryan Struyk, Ben Gittleson, Ines DeLaCuetara and Jessica Hopper contributed to this report.