Mitt Romney spoke at the University of Utah in Salt Lake City on March 3. (AP/Rick Bowmer)

Former Republican presidential nominee Mitt Romney is now playing an even more active role in the 2016 campaign: He's working the phones.

On Monday, Romney recorded a phone call paid for by the presidential campaign of Sen. Marco Rubio (R-Fla.) that is being sent to Republicans in four states voting on Tuesday — Michigan, Mississippi, Idaho and Hawaii. In the brief message, Romney makes clear that the call was paid for by the Rubio campaign, but he stops short of endorsing the senator. On Tuesday, he recorded a similar call paid for by Ohio Gov. John Kasich's campaign that will reach Republicans in Michigan.

"Tomorrow you have the opportunity to vote for a Republican nominee for president. I believe these are critical times that demand a serious, thoughtful commander-in-chief. If we Republicans were to choose Donald Trump as our nominee, I believe that the prospects for a safe and prosperous future would be greatly diminished — and I’m convinced Donald Trump would lose to Hillary Clinton. So please vote tomorrow for a candidate who can defeat Hillary Clinton and who can make us proud," Romney said.

Romney used virtually the same script in the calls paid for by Kasich.

News of the robocall was first reported by the New York Times.

A Romney spokeswoman said in an email that the former Massachusetts governor "has offered and is glad to help Senator Marco Rubio, [Texas] 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 Rubio campaign didn't return requests for comment. Kasich's campaign confirmed that the calls by Romney were placed. After his final Michigan campaign event today, at a brewpub in Lansing, Kasich expressed gratitude for the calls without endorsing their entire message.

"Well, look – the situation is, you know what I’m for," said Kasich. "I’m for the things that I know, and for the things I have done to help lift the country. Gov. Romney’s kind of recording robocalls for everyone, and I didn’t want somebody to think he favored one person over me, because he doesn’t. It’s his words, I don’t write his scripts."

Asked if he was distancing himself from something his campaign paid to distribute, Kasich said he was only supporting the idea that voters in upcoming states stick with him, instead of bolting to someone like Cruz.

"Well, look, we want to make sure people don’t think that Romney’s for someone else and not for me, particularly in the state of Michigan," said Kasich. "So, is this the scenario that I would have liked, to have it work out like this? Not really, but it is what it is."

Even though Romney isn't endorsing a candidate, his continued participation in the race is unprecedented. Never before has a party's immediate past nominee later meddled in its affairs so boldly.

Romney reemerged on the political scene last week by delivering a point-by-point indictment of Trump, the GOP front-runner, warning Republicans that nominating the New York businessman would be detrimental to the party and the country.

He called for a scenario that likely would lead to a convention floor flight, recommending that voters cast ballots for Rubio in his home state of Florida, for Kasich in Ohio, and for the candidate best positioned to deny Trump a win, everywhere else.

“The only serious policy proposals that deal with a broad range of national challenges that we confront today come from Ted Cruz, Marco Rubio and John Kasich,” Romney said in his speech.

In a speech in Salt Lake City March 3, former Republican presidential nominee Mitt Romney denounced support for candidate Donald Trump, saying Trump "is playing the members of the American public for suckers." Here are key moments from that speech. (Sarah Parnass/The Washington Post)

“I know that some people want this race to be over,” he added. “They look at history and say a trend like Mr. Trump’s isn’t going to be stopped. Perhaps. But the rules of political history have pretty much all been shredded during this campaign.”

Romney urged Cruz, Kasich and Rubio to “find some common ground” and help the party “nominate a person who could win the general election and who will represent the values and policies of conservatism.”

So far, all three have rebuked such suggestions.

Throughout the speech Romney repeatedly dismissed Trump’s personal behavior: “Think of Donald Trump’s personal qualities. The bullying. The greed. The showing off. The misogyny. The absurd third-grade theatrics.”

Trump fired back, bemoaning Romney's "nasty" remarks and dismissing him as a "choke artist" and "failed candidate" who was eager for Trump's endorsement in 2012.

Romney's more active involvement comes as party leaders now believe that Trump is vulnerable and that a continued blitz of attacks could puncture the billionaire mogul’s support and leave him limping onto the convention floor.

In private conversations in recent days at a Republican Governors Association retreat in Park City, Utah, and at a gathering of conservative policy minds and financiers in Sea Island, Ga., governors, lawmakers, top party donors and operatives are learning about plans to systematically chip away at Trump's delegate lead.

As that work continues, House Speaker Paul D. Ryan (R-Wis.) — Romney's 2012 running mate — held separate phone conversations with Trump and Cruz to fill them in on House Republicans' legislative plans for the remainder of the year.

This post has been updated.