Froma Harrop is a syndicated columnist. Follow her on Twitter: @FromaHarrop . The opinions expressed in this commentary are hers; view more opinion articles at CNN.

(CNN) Democrats might ask themselves why President Donald Trump is so intent on smearing former Vice President Joe Biden with phony scandals. Why isn't he doing the same to Sen. Elizabeth Warren, who's been climbing in the polls and has a shot at becoming the Democratic nominee for president?

The clear answer is that Biden, in his own words, could "beat him like a drum" -- and while Warren could win against Trump, her victory does not look like a resounding one, according to recent polls . In Trump's view, Biden poses the greatest threat and must be destroyed. And what better way than to wound Biden to the point that Democrats might think he's more trouble than he's worth?

Warren fans will point to national polls showing their candidate defeating Trump in a general election. But it's important to note that Warren has been largely spared from Trump's attacks thus far, and those polls don't take into account the campaign of calumny that would surely follow a Warren nomination. It's Biden that Trump clearly most fears.

It would not surprise me if foreign propagandists decided to take to social media to advance Warren's candidacy and hurt Biden's. This happened in 2016, when Russian trolls tried to turn fans of Bernie Sanders against Hillary Clinton. Many supporters took the bait and either voted for third party candidates or sat out the election.

Though polls suggest that Warren could defeat Trump by a few points, Biden still holds a more comfortable margin over the President. Let's not forget that poll numbers don't necessarily translate to an Electoral College victory. In 2016, Clinton lost the election even though she bested Trump by nearly 2.9 million votes nationally.

Biden is still a stronger bet against Trump in crucial swing states like Wisconsin and Ohio, compared to Warren. The most recent Wisconsin poll pitting Democrats against Trump has Biden winning by 9 points, while Elizabeth Warren is tied with the President. In Ohio, Biden is the only Democrat who can beat Trump. According to the most recent Quinnipiac poll from July, Biden leads Trump by 8 points, while Sanders and Warren both trail the President by 1.

President Trump has claimed Biden called for Ukraine to fire its top prosecutor who was "investigating his son." But there is no evidence Hunter Biden was under investigation. The probe was into the business dealings of the owner of a Ukrainian natural gas company Burisma Holdings, where Hunter Biden sat on the board of directors.

How should Democrats handle Trump's hatchet job on Biden's integrity? For starters, they should not get sucked into arguments over Biden's guilt or innocence. There is no evidence of wrongdoing by either Joe or Hunter Biden, and Ukraine's current prosecutor general also came out in defense of the two. And making the case that the Trump children are profiting from their proximity to their father establishes a false equivalence that turns a flat-out lie into a "what about the other side?" discussion.

Meanwhile, other polls underscore weaknesses in the Warren campaign. For one thing, Warren seems to have a likeability problem, even in politically friendly states. A CNN/UNH Survey Center poll from July asked likely New Hampshire voters which Democrat they found most likeable. Biden and Sanders topped the list at 20% each, while only 4% chose Warren.

Then there are the policies. Warren endorses a Medicare for All health plan that would do away with private coverage, even though a Wall Street Journal/NBC News poll shows only 41% of registered voters supporting a system that eliminates private health coverage.

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Biden's support for a government-run option in Obamacare is much closer to the public's preference. The poll found that 67% want an option to buy health coverage through a Medicare program, while keeping the possibility of private insurance.

Warren's call for decriminalizing illegal border crossings is also widely unpopular. She might be able to finesse that position in a general election campaign, but her views on immigration generally have ventured dangerously close to open borders.

Democrats might ultimately choose someone other than Biden as their presidential nominee. But it should be their decision and not a scared response to Trump's manipulation of the truth. There is a good reason he's so intent on crushing Biden.