Joe Manchin, one of the few threats to Kamala Harris, says he won't take her on

• Sen. Joe Manchin rules out presidential bid, will not challenge VP Kamala Harris.
• But Manchin, now an Independent, said he believes Democrats' rapid support for Harris is a mistake.

Sen. Joe Manchin of Virginia, who briefly sparked fears among Democrats that he could re-register as a Democrat and challenge Vice President Kamala Harris for the presidential nomination, has ruled out running.

Appearing on CBS Mornings on Monday, Manchin, who left the Democratic Party and filed as an Independent in May, said: "I am not going to be a candidate for president."

He added: "I am a candidate for basically speaking for the middle of this country."

Manchin later explained his reasoning for not challenging Harris, saying: "I don't need that."

Although he won't oppose Harris, who already has substantial support within the party and an endorsement from President Joe Biden, Manchin argued that rallying behind Harris so quickly was a "mistake."

He said he believes that the party's apparent decision not to pursue open competition via a primary or a "mini-process" was misguided.

"We're not going to be able to see a vigorous, even a three-week primary process," Manchin added.

The party needs to have a candidate in place in a matter of weeks, with the nominating convention taking place next month.

On Sunday, shortly after Biden announced he was dropping out, Harris quickly emerged as the frontrunner to replace him to face former President Donald Trump in November.

According to Bloomberg and other media outlets, Jonathan Kott, who previously served as Manchin's communications director, said the West Virginia senator was "seriously considering" running for president as a Democrat.

On Sunday, before Biden's announcement, Kott told CNN that Manchin said he would prefer an "open process."

He named Govs. Andy Beshear of Kentucky and Josh Shapiro of Pennsylvania — both considered running-mate material for Harris — as potential leaders.

Though Manchin would have faced an uphill battle for the nomination, it could have complicated a process many Democrats hope will be quick and decisive.

For now, those anxieties appear to have been quelled.