Democratic presidential candidate U.S. Sen. Bernie Sanders (I-VT) wipes sweat from his forehead during an interview at Iowa State Fair Justin Sullivan/Getty Images Sen. Bernie Sanders (I-Vermont) is continuing to rise in Democratic primary polls, but continued trends show he's unlikely to become a real threat to Hillary Clinton for the Democratic nomination.

According to a new CNN poll released Wednesday, Sanders has peeled away at some of Clinton's support over the past month. Since CNN conducted its last poll in mid-July, Sanders has jumped 9 points to 27% support. Clinton, meanwhile, has fallen 9 points to 48%.

He's attracting the largest crowds of the 2016 race. And other polls have shown that he could realistically have a shot at winning the first-in-the-nation primary state of New Hampshire, a potentially significant coup.

But despite Sanders' rising popularity, trends in the CNN poll indicate he's not any closer to actually winning the nomination.

"Bernie Sanders will not be the nominee," said Greg Valliere, the chief political strategist at the Potomac Research Group, in a note Wednesday. "Party leaders are convinced he would lose 49 states."

One number in particular, from a separate a CNN poll taken in late June, showed that only 2% of Democratic voters think Sanders has the best chance of winning the general election. That number is a problem for Sanders, as most voters want to cast their vote for a winning candidate. They'll often cast a reluctant vote for their second choice if they perceive the candidate to have a better chance of winning, studies have shown.

And the new CNN poll, though it didn't test the electability question, found that unless Sanders can significantly increase his standing among Democrats who are currently not considering him at all, he might be hitting a ceiling.

Clinton is the second-choice selection of 26% of Democratic voters, while Sanders is the second choice pick of only 16%.

The poll also tested Vice President Joe Biden, who is newly pondering a challenge to Clinton. Biden is the second choice of 37% of Democratic or Democratic-leaning voters. But if Biden decides not to run, the poll displayed that Clinton would absorb more of his support than Sanders. According to the poll, with Biden out, Clinton would sit at 56% to Sanders' 33%.

U.S. Vice President Joe Biden (L) speaks next to Secretary of U.S. State Hillary Clinton at a State Luncheon in honor of the State Visit of South Korea's President Lee Myung-bak at the State Department in Washington October 13, 2011. REUTERS/Yuri Gripas

The results suggest that Sanders' fundamental challenge to winning the nomination is unchanged: He appeals to predominantly white voters in liberal enclaves where he's drawn those large crowds.

Sanders himself has admitted that he's not well-known in minority communities, a fact that likely hasn't been helped by his somewhat awkward encounters with protesters from the "Black Lives Matter" movement.

"If Sanders wins Iowa or New Hampshire, it will build a lot of momentum for him that will help in the states that follow, but he's still going to struggle in places like South Carolina with large black populations and Nevada with large Hispanic populations unless he improves his appeal to nonwhite voters," Tom Jensen, the director of the firm Public Policy Polling, told Business Insider recently.

Jensen said most recent polls put Clinton's support among African-American voters at around 70% to 80%.