Mitt Romney is urging Americans to donate to Florida Sen. Marco Rubio's re-election effort.

As one of the most prominent anti-Trump Republicans, Romney was rumored to have supported the Florida senator's presidential bid but never formally endorsed Rubio. The 2012 GOP nominee has sharply criticized Donald Trump, and is reportedly considering boosting an independent or third party candidate at the presidential level.

But on primary day in the Sunshine State, the Rubio campaign distributed a letter from Romney urging Republicans to donate to the senator's cause.

"I am committed to doing my part to protect our conservative Senate majority by working to re-elect Marco Rubio to the United States Senate," Romney wrote in the fundraising solicitation. "Over the last few years I have gotten to know Marco. Let me tell you, he is one of the most impressive public servants I've had the opportunity to support. Ever since Marco was elected back in 2010, he has been a stalwart example of what conservative leadership in Washington should look like."

The first-term senator, who reversed his intention to leave the U.S. Senate, is facing a primary challenge from Carlos Beruff, a businessman who supports Trump, in Tuesday's primary. Rubio left the presidential race after Trump's victory in Florida, but Rubio is expected to triumph on Tuesday.

Rubio's shifting statements about Trump — alternatively opposing and supporting the GOP nominee — could prove more consequential in the remaining 69 days before the general election. Whether the Rubio campaign's decision to spotlight Romney's support indicates that the Florida Republican will gravitate away from Trump for the general election campaign remains to be seen.