On a call to about 20 potential Rubio donors on Wednesday, Romney said that given the likelihood of victory for Hillary Clinton in November, it was important to devote financial resources toward holding seats like Rubio's to serve as a check to a Democratic president, according to a recording of the conversation obtained by CNN.
"The unfortunate truth about this election is that nobody's really happy -- I shouldn't say nobody -- most Americans are not really happy with either outcome, either the Republican or Democrat outcome. But right now, Hillary's in the lead," Romney said.
He added: "And if she were to win by too large a margin it would be very, very hard for us to be able to maintain the principles to keep America the nation that it is. It's important that regardless of what happens on the presidential level, that we have Sen. Rubio able to be successful and win in Florida so we can maintain our majority in the Senate."
As a presidential candidate earlier this year, Rubio initially said he wouldn't run for re-election to the Senate, but he changed his mind after dropping out of the White House race in June.
Romney, who lent support for Rubio before he left the presidential race, was critical of Trump throughout the presidential primary cycle, and he has refused to endorse him after he clinched the 2016 GOP presidential nomination.
Rubio has tepidly endorsed Trump, despite calling him a "con man" during the primary.
But Trump has plummeted in key battleground state polls since the party convention, causing fear among Republicans that his fall in favor could affect House and Senate races.
In Florida, a Monmouth University conducted this month found Clinton leading Trump by nine percentage points. In his own race, Rubio is likely to face Democratic Rep. Patrick Murphy in the fall. A Quinnipiac University survey of the Florida Senate race shows Rubio edging out Murphy by a margin of just 48-45 percentage points.
With 24 Republican senators up for re-election this fall, Democrats know they have an opportunity to take control of the upper chamber. Should Clinton win the presidency, the party would only need to pick up four seats to regain the majority, a reality that has Republicans scrambling to defend their power.
On the call, Romney also praised Rubio as a lawmaker open to compromise with Democrats, calling him "a voice of reason and thoughtfulness" who can "find relationships on our side of the aisle and across the aisle to get some things done."