Observers expect the relationship between Marco Rubio and his former mentor Jeb Bush to further disintegrate tonight, as the two Republican presidential candidates are likely to attack each other fiercely during their party's fourth presidential debate.

According to NBC News, the former Florida governor desperately needs to appear strong in this showing, and he will likely focus on Rubio, who -- along with his Texas colleague Sen. Ted Cruz -- has emerged as a leading "mainstream" candidate within the Republican Party. The two senators tied for third place in a recent national poll conducted by the Los Angeles Times and the University of Southern California, though they still trail "renegade" front-runners Donald Trump and Ben Carson.

Ahead of the debate, set to take place at 9 p.m. EST and hosted by the Fox Business Network, a group supporting Bush's run for president has gone so far as to attack Rubio on social issues, challenging the Florida senator's stance on abortion, The New York Times reported. In a provocative video, the Right to Rise super PAC, which has raised more than $100 million, questions Rubio's electability because of his hard-line view on the issue, the newspaper detailed.

Meanwhile, both Bush and Rubio seem to be trying to score an endorsement from Scott Walker, a former rival who dropped out of the race in September, according to NBC News. The Fox Business Network debate will be held in Wisconsin, where Walker has served as governor since 2010.

Rubio and Walker met on Monday night at a fundraising event in Burlington, Wisconsin, an unidentified campaign source told the network. Similarly, Bush appeared with the governor at a school choice event on Monday. Both candidates have suggested that, while they have not expressly asked for Walker's endorsement, they would like to count on the Wisconsinite's support.

Viewers who want to tune in to see Bush and Rubio face off, along with the remaining GOP White House hopefuls, can watch a livestream of tonight's debate on the host channel's website.