When he takes the debate stage in Milwaukee Tuesday night, Marco Rubio will be addressing two audiences.

One will be the voters whom the surging presidential candidate needs to do well in early voting states such as Iowa and New Hampshire. The other will be the deep-pocketed donors, some of whom have stayed on the sidelines as the Republican race has progressed, who Rubio needs to mount a large-scale national campaign.

To date, Rubio has been a middle-tier contender, both in his showing in the polls and his campaign contributions. He is not leading in any early-voting state and has lagged other candidates such as Jeb Bush and Ted Cruz in terms of fund-raising.

But after his break-out performance in the last Republican debate, another strong debate performance could change that. For donors, a debate can be much valuable than highly scripted campaign events to measure how a candidate stands up to scrutiny, fends off barbs and frames issues.Strategists and donors say the pressure is on Rubio to show that he is ready to unseat Bush, who has tumbled in the polls, as the first choice among the Republican establishment.

"Right now, the establishment money guys are wondering, If not Bush, then who?" said Saul Anuzis, a former chair of the Michigan Republican Party who is backing Cruz. "Rubio has to show the leadership establishment types that he would be a good alternative to Bush."

Added Ron Bonjean, an unaffiliated Republican strategist: "This could be a make-or break performance for Bush among key donors."

Because of his relative youth, Cuban-American background, and telegenic appeal, Rubio, 44, has long been viewed by pundits as perhaps the party's best hope to defeat the likely Democratic nominee, Hillary Clinton.

And since the last Republican debate, when Rubio, a first-term U.S. senator, was widely praised for his command of the stage, there is a sign that a shift is taking place. Paul Singer, a hedge-fund billionaire who pumped millions into the 2012 elections, came out for Rubio late last month. Then Brian Ballard, a top Florida fundraiser, defected from Bush's camp to Rubio's.

"There are a lot of people in Jeb's world who are concerned right now," said a source close toRubio's campaign. "If Jeb is not the nominee, they want to see Marco do well."

Several other big-money donors from the last presidential election have also been sitting out this one. Chief among those is casino magnate Sheldon Adelson, who supported Newt Gingrich's 2012 bid with a massive infusion of cash into the political action committee backing the former House of Representatives speaker.

R.C. Hammond, a former aide to Gingrich, said a strong night by Rubio on Tuesday could help win over Adelson and other donors who could then help fortify the super PAC aiding him. "If I'mRubio, I'm looking for a super PAC donor to step forward and say I got your back," Hammond said.

Such committees can spend millions on ads defending a candidate and attacking his rivals.

This week, a super PAC supporting Cruz released an ad ripping Rubio and his support for a Senate immigration-reform measure. And while Cruz is not expected to go after Rubio directly at Tuesday's debate, his campaign told Reuters that Cruz, a U.S. senator from Texas, if given the chance, will draw a contrast between his positions on issues such as immigration and Rubio's.

For his part, Bush is likely to continue to highlight Rubio's relative lack of political experience as compared with his record as a two-term Florida governor. Bush's attempt to criticize Rubio in last month's debate, however, backfired when Rubio got the best of the exchange.

Bush is running out of chances to change perceptions of his candidacy.

"Given that the next debate is not until Dec. 15, I think most of the unattached donors on the Republican side are looking to make a commitment in the next month," said one high-level Rubiodonor, who asked not to be named. "I think a lot of donors wrestled with his inexperience and his youth, but now they see him as best suited in the race."

Rubio raised just $6 million during the third-quarter, far behind Bush, Cruz and Ben Carson. And two committees supporting Rubio, both dubbed Conservative Solutions, lag behind those backing Bush and Cruz.

The economic-themed debate in Milwaukee, moderated by the Fox Business Network, likely will play to Rubio's strength, whose campaign has focused on the "sharing economy," higher-education reform and a proposal for a new tax credit for families.

"It's prime-time," Hammond said. "This is when the winners are going to be defined."