A new poll set for release Monday shows former Florida Gov. Jeb Bush (R) lagging in a theoretical general-election matchup against Hillary Clinton, while Sen. Marco Rubio (R-Florida) is the only Republican candidate with a slight edge over the former secretary of state.

The new national poll from Public Policy Polling, shared exclusively with Business Insider, found Bush faring worse than any other major Republican candidate surveyed in a theoretical matchup with Clinton, the Democratic front-runner.

The PPP survey showed Clinton would lead Bush, 44% to 39%, in a theoretical general-election matchup, while 17% said they're still undecided.

Sen. Ted Cruz (R-Texas) comes a bit closer against Clinton, garnering 43% support to Clinton's 45%, with 12% of voters undecided. Clinton ties retired neurosurgeon Ben Carson, 45% to 45%, while edging current Republican front-runner Donald Trump by a 46-43 margin.

Rubio, meanwhile, fared best among the candidates surveyed in the poll. He garnered 44% support to Clinton's 43%, with 13% of surveyed voters undecided.

The poll illustrates another obstacle for Bush, who once said the Republican nominee would have to "lose the primary to win the general" election. Despite his more moderate position on immigration relative to Trump or Cruz, the poll still found Bush performing — albeit within the margin of error — in the general election at this point.

What's dragging Bush down: He's the least-favorable Republican candidate in the field. Overall, just 24% of voters view Bush favorably, according to Public Policy Polling, while 53% see him in an unfavorable light. That's a worse net-favorability score than even Trump, who is viewed unfavorably by a 35-56 split.

Bush is attempting to regain some support among Republican primary voters by painting himself as a more "serious" candidate and sparring directly with Trump, putting foreign policy and national security front-and-center in his bid for the presidency.

The Washington Post reported this summer that Bush told an audience at a fundraiser that by "December 15, Trump will be in decline."

Indeed, since that time it appears that the opposite has occurred. He has seen his position sink in polls since mid-July, when polling averages found his support at above 17%. He's now in mid-single digits nationally. In the last month, however, his support rose slightly to 7% compared to the previous month's PPP survey.

Publicly, many in Bush's orbit say they are completely unconcerned by former governor's stagnating poll numbers.

"The race is immaterial. They’re starting to figure out — who do I want to be president, and who do I want to see in the Oval Office? Who do I want to be commander in chief?" former Department of Homeland Security Secretary Tom Ridge told Business Insider earlier this month. "So that's why I think he’s gaining momentum. I don’t care what the polls say. I’ve never been a big believer in national polls."

But privately, some prominent Republicans who had expressed support for Bush have cast doubt on his comeback chances.

"The race is turning into a three man contest — Trump (outsider lane), Cruz (conservative lane), Rubio (establishment lane) with [New Jersey Gov. Chris] Christie trying to dislodge Rubio in the establishment lane with a win" in New Hampshire, a former senior adviser to 2012 Republican nominee Mitt Romney told Business Insider last week.

Beyond the Republican race, Monday's poll also showed the potential weaknesses of Sen. Bernie Sanders (I-Vermont) in the Democratic field.

Despite his impressive grassroots fundraising hauls and the massive crowds that attend his rallies, it's still no surprise that Clinton has touted her chances in a general election. She fares slightly better in several head-to-head matchups with top-tier Republican candidates like Bush, Cruz, Trump, and Rubio.

View the full poll below: