Washington (AFP) - Democratic frontrunners Joe Biden and Bernie Sanders are trading barbs, each accusing the other of lying and having a smaller chance of defeating Donald Trump in November, as the race for the party's nomination tightens.
Biden holds the lead in national polling averages, but Sanders has been on the rise since December.
A new CNN/SSRS poll puts Sanders in first place with 27 percent, within the margin of error but slipping ahead of Biden, who came in at 24 percent.
Results from another survey Wednesday by Morning Consult were virtually the inverse: Barack Obama's former vice president scored 29 percent to 24 percent for the US senator from Vermont.
As the race has narrowed, the two septuagenarians -- Biden the moderate, Sanders a self-described "Democratic socialist" -- have grown testy, accusing each other of dishonesty.
The change in tone recalls the bitter 2016 primary battle between Sanders and Hillary Clinton, who in the end won the Democratic nomination but lost the election to Trump.
"I find it amazing that we go back and look at statements, many of them, most of them, taken out of context of 10, 20, 30, 35 years ago," Biden said on MSNBC Wednesday.
The day before, Sanders had accused Biden in a tweet accompanied by a video of having long wanted to cut funding for Social Security, the US public pension system.
"Let’s be honest, Joe. One of us fought for decades to cut Social Security, and one of us didn’t. But don’t take it from me. Take it from you," he wrote.
A Sanders spokeswoman, Briahna Joy Gray, piled on, posting a long video comparing the two candidates' positions, and concluding that Biden's record on foreign or trade policy would crumble under Trump's attacks if the two were to face off one-on-one.
Then she called on the former vice president to "Tell the truth Joe," invoking a hashtag popular with Sanders supporters.
In an ad posted on Twitter by his account, Biden had previously taken a shot at Sanders.
"Bernie’s negative attacks won’t change the truth," a narrator says. "Joe Biden is still the strongest Democrat to beat Donald Trump."