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.