Former Iowa Gov. Tom Vilsack said he will not run for the U.S. Senate against Sen. Joni Ernst in 2020 because he's not prepared to fully commit to another run for office.

"I didn't feel that I could make that commitment," he told the Register in a brief interview Friday night. "And without that commitment, I would be doing a disservice to the party, my family and myself."

Vilsack spoke with the newspaper after he introduced Colorado Sen. Michael Bennet to an audience in Waukee. Iowa operative and long-time Vilsack aid Matt Paul confirmed to the Register his decision not to run earlier Friday.

Vilsack, who has a long history in Iowa and national politics, was seen by many Democratic activists as the party's most prominent would-be challenger to Ernst. In recent months he has downplayed speculation that he might enter the race but had not sworn off the possibility completely.

Vilsack said that among the reasons he will not run are his age — the former governor is 68 years old — and his desire to allow other Democrats the opportunity to do so.

"I'm confident there are going to be a lot of people stepping up that will have the commitment, the desire and the passion that it takes to win, and I just didn't think I had it," he said.

► Iowa Poll: Joni Ernst's approval rating hits new high as she readies for 2020 re-election bid

In a Des Moines Register/Mediacom Iowa Poll this month a majority of Iowans— 53 percent — said they have a favorable view of Vilsack, who has a long history in the public eye. Vilsack also was U.S. secretary of agriculture under President Barack Obama and currently is the CEO and president of the U.S. Dairy Export Council.

Sixty-nine percent of Democrats view Vilsack favorably along with 51 percent of independents and 48 percent of Republicans.

Ernst, a military veteran from Red Oak, is viewed as a formidable incumbent. Her approval rating hit its highest point ever this month, according to the poll.

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Fifty-seven percent of Iowans say they approve of the job she’s doing — a 10 percentage-point increase since September. She earns approval from a majority of Iowans in each of the state’s four congressional districts.That includes a high of 65 percent in the Republican-heavy 4th District, in northwest and north-central Iowa, and a low of 51 percent in the Democratic-leaning 2nd District, in southeast Iowa.

Ernst became the first woman in Iowa elected to either chamber of Congress in 2014. That year, she defeated Democrat Bruce Braley to succeed retiring U.S. Sen. Tom Harkin, a veteran Democrat.

She announced in December that she intends to seek re-election.

"I am running again,” she told reporters. “So to anyone who would like to step up and enter into that race, I say: ‘Bring it on.’”

No challenger, Republican or Democrat, has yet entered the race.

"Secretary Vilsack has been — and continues to be — an outstanding public servant both for the people of Iowa and our nation. He would have been a tremendous advocate for Iowa in the Senate and we appreciate his thoughtful and deliberate consideration of making a run for this seat," said Iowa Democratic Party chair Troy Price. "We’re confident that we will have a spirited primary season and show the people of Iowa that our party is just as strong as this great state."

Other Democrats who might consider a run include 2018 4th District congressional candidate J.D. Scholten and 2018 3rd District candidate Theresa Greenfield. Neither yet has the name recognition of Vilsack, the Register's poll showed.