Former Vice President Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE will be the clear Democratic front-runner for the presidential nomination when he enters the race, but there are lingering doubts about his ability to win a primary as the Democratic Party moves to the left.

Those questions resurfaced last week when progressives became angry over his comments saying Vice President Pence is a “decent guy.”

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Separately, a CNN focus group earlier this week rejected Biden outright, with some prospective voters in the group saying the party needed “bold, strong leadership,” as one put it, “and you’ll find that in the progressives.”

Biden remains a huge figure in the race even before entering it.

After eight years as former President Obama’s vice president, Biden has enormous goodwill built up within a party that remembers his time in the administration fondly.

He’s also seen as a major threat to President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE in the battleground states of Wisconsin, Michigan and Pennsylvania — where Biden was born. If a Democrat in 2020 can win those three states and carry the states won by the party’s 2016 nominee, Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE, they will win the presidential election.

“He resonates with blue-collar workers and he also has the ability to turn out minority voters because of his time with President Obama,” said Republican strategist Shermichael Singleton. “And if he selects a Kamala Harris Kamala HarrisButtigieg stands in as Pence for Harris's debate practice First presidential debate to cover coronavirus, Supreme Court Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE or a Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE [as his running mate], then I think it would almost be a done deal, period.”

Republicans certainly see Biden as a rival who could unseat Trump.

Conservatives gathered at the Conservative Political Action Conference said Biden would pose the biggest threat to President Trump in the 2020 general election.

Republican super PACs are already attacking Biden behind the scenes, a preview of an avalanche of negative research likely to be unloaded on the former vice president if he does enter the race.

The whispered and nonwhispered attacks can be interpreted as a sign of the respect Biden has from Republicans as a real threat in 2020.

None of Biden’s obvious strengths have erased the real doubts about his candidacy, and he will face a seemingly endless flood of questions about his fundraising, his propensity for gaffes, his age and his centrist politics if he enters the race.

For many critics that will begin with the nice things the 76-year-old is willing to say about Republicans such as Pence.

“His worldview on dealing with Republicans and most issues are out of step with where a lot of Democratic voters are right now,” said one Democratic strategist. “The problem Biden has to overcome isn't a problem with the Pence remarks themselves, but rather they're an early warning sign of what he is going to face when he goes from senior party figure and lovable grandpa Joe to an actual candidate.”

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Democratic strategist Christy Setzer said it’s “ironic” that Biden finds himself in this scenario because when issues like same-sex marriage surfaced in the Obama White House, “it was Biden who was seen as moving Obama forward on issues.”

And while Biden frequently touts his willingness to work with Republicans, “sleeping with the enemy, in this case, being chummy with Trump loyalists, won’t be taken lightly by Democratic primary voters,” Setzer said.

“No one’s saying Biden has to protest outside of Tucker Carlson’s house, but praising Pence was an unforced error,” she said.

Democratic strategist Basil Smikle added that in order to prevail in the primary, Biden will have to “convince the left that he’s going to undo all that Trump has done and lay a path for a progressive future that incorporates many of the ideas promoted by the winners of the midterms.”

Republican groups are getting ready to shine a light on what they perceive as Biden’s blemishes.

On Thursday, after CNN ran a piece about Biden’s 1993 Senate floor bill where he pushed the controversial crime bill, America Rising, the Republican super PAC pushing opposition research, highlighted that Biden warned of “predators on our streets” who were “beyond the pale.”

The Republican National Committee (RNC), which has been compiling opposition on Biden for more than a year, will focus primarily on Biden’s time as vice president.

“The big thing with Biden is he's going to have to answer for all those failed Obama policies,” said Michael Aherns, an RNC spokesman.

Officials there say the left wing of the party will ironically play an ally of sorts when they dig into Biden’s Senate record, including the crime bill, the vote for the war in Iraq and his comments to Anita Hill when he presided over the Senate Judiciary Committee.

David Wade, who served as a spokesman for Biden during the 2008 Obama-Biden campaign, said the scrutiny was always inevitable.

“Everyone gets their time in the barrel,” Wade said. “When you’re the biggest name and early front-runner, you get shot at first.”

“Ultimately primary contests are a choice between candidates, each with their own vulnerabilities, not a referendum on a single candidate,” Wade said, “and Democratic voters are going to settle on a nominee who best connects with their values and satisfied their deep desire to beat Trump.”