A top supporter of Hillary Clinton struggled to explain on Thursday why the Democratic presidential hopeful is failing to capture Americans' overwhelming support considering she's running against a "huckster and a conman."

During an early morning appearance on MSNBC, Missouri Sen. Claire McCaskill was asked why Clinton isn't enjoying a larger lead in national and battleground state polls when her Republican opponent, Donald Trump, has refused to release his tax returns, lacks the "same depth in foreign policy [knowledge]," and now faces questions about his charitable foundation's foreign business ties.

"I think in presidential elections, history has shown that the American people do have a penchant for a new, bright and shiny object," she responded. "There is a lot of dissatisfaction in this country and a sense that we need to blow up the system, and who better to blow up the system than a huckster and a conman?"

McCaskill then added, "I will say some of it is inexplicable to me, honestly, candidly."

The Missouri Democrat, who endorsed Clinton's 2016 White House bid three years before the former secretary of state announced she was running, said the Clinton campaign "always knew this [election] was going to be close," which is the principal reason why they built such a formidable ground operation.

But Democrats still have to "mobilize these voters," she said. "People need to quit wringing their hands and get to work."

Clinton's post-convention lead over Trump has diminished in recent weeks, following new revelations about her private email practices and the Clinton Foundation.

A Quinnipiac Poll released Wednesday found the former first lady leading Trump by just 5 points among likely voters nationwide, down from her 10-point lead in late August.