Labor Day brings not only the next phase of the marathon 2020 presidential race, but also the greatest threat to a shakeup in the contest so far.

On the Republican side, President Trump’s political standing remains remarkably consistent. His approval among GOP voters is overwhelming and as strong as ever. Even his latest quixotic possible primary challenger, former South Carolina Gov. and U.S. Rep. Mark Sanford, has already admitted he can’t beat Trump, saying he’s not “delusional on prospects” of winning.

It’s a different outlook on the Democratic side, where unlike Trump, former Vice President Joe Biden faces real challenges to his front-runner status.

BIDEN, STILL LEADING, BELIEVES HE CAN SURVIVE ANY NEW GAFFES, DEM POLLSTER SAYS

Biden’s lead has been both wide and enduring. He has not trailed in a single public primary poll since entering the race in April. In fact, he leads by 11 points in the Real Clear Politics average.

That’s why the since-discredited recent Monmouth University poll showing Biden in a three-way tie got so much attention from a salivating press corps and an outraged Biden camp. The response from Team Biden was swift and severe, dismissing it – correctly it turns out – as an outlier.

Members of the Biden campaign know that a poll showing their candidate slipping fulfills the media’s desired prophecy of political gravity catching up to an unsteady candidate who has shown signs of his 76 years.

Even the bombshell Washington Post story about Biden telling a “moving but false war story” isn’t the biggest iceberg looming in his path. While the Post’s facts were damning, exaggerations about time spent in war zones are not the career-ender they once were.

Four years after NBC’s Brian Williams tall tale about a helicopter in Iraq cost him his job as anchor of the “NBC Nightly News,” he has a late-night show on MSNBC.

Besides, Biden has made a career of braggadocio and bombast. For voters, it’s an ingredient already baked in the cake.

In 1988 – before plagiarism ended his candidacy for the Democratic presidential nomination – Biden was caught on camera dressing down a New Hampshire voter with the boast, “I think I probably have a much higher IQ than you do,” before going on to exaggerate his law school GPA.

It’s in Houston where Biden has a problem. The Texas city is the home of the Sept. 12 Democratic presidential candidate debate. The three-hour debate will be the first time the former vice president faces his biggest threat to date: Sen. Elizabeth Warren, D-Mass.

Only 10 candidates will debate, all on one night. That’s a contrast to earlier debates that broke candidates into two groups of 10 candidates each, broadcast over two nights.

In addition to nipping closest at Biden’s heels, Warren is the candidate with the longest record of disdain toward him.

The two have tangled publicly in the past, most famously over legislation involving bankruptcy when Biden was a senator from Delaware and Warren a little-known law professor. Their exchanges were so heated that when Biden was swearing Warren into the Senate years later, he told her: “You gave me hell.”

At issue then is the same issue debated now: the direction of the Democratic Party. Biden represents a throwback to the Bill Clinton era of the 1990s – push left-of-center priorities when possible. but also look to cut deals with Republicans.

But as Axios reported: “Hillary Clinton and former President Obama would sound like conservative Democrats in this field.”

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Warren – and her ideological comrade Sen. Bernie Sanders, I-Vt., – are the main culprits for that shift. On virtually every big-ticket issue impacting Americans’ lives – taxes, energy, health care, college, even the types of cars people are allowed to drive – Warren and Sanders have pursued an uncompromising and unapologetic far-left policy agenda.

Both Warren and Sanders share a belief in a more intrusive government. They sneer at the Clinton era as a time when Democrats “sold out” instead of pushing for "big, structural change," as Warren is fond of saying.

Unlike Biden, Warren is in her element on the debate stage. Where he stammers and stumbles, she relishes cutting down rivals under the klieg lights. In Houston, for the first time, she will get to face Biden in her natural setting.

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Lost amid the discussion of the polls is projecting ahead to what happens when Sanders realizes lightning can’t be captured twice and exits the race. With their shared nearly identical platform, it stands to reason that Warren will gain the majority of Sanders’ supporters.

All of a sudden, that three-way cluster atop the polling pack could turn into a wide Warren lead, and Biden could be left yearning for the dog days of summer.

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