Progressives unhappy that Sen. Dianne Feinstein is running for a sixth term might be pleased to learn that she could soon have two more liberal challengers, as billionaire San Francisco environmentalist Tom Steyer is strongly considering a run and state Senate President Pro Tem Kevin de León, D-Los Angeles, could jump in as soon as next week.

Sources close to Steyer confirmed Thursday that he is “very much looking at the Senate seat.” But if Steyer enters next year’s Senate race along with his friend de León, analysts and progressive leaders worry that there might not be enough liberal oxygen for both men to consume — and that ultimately could benefit the more centrist Feinstein.

If their goal is to provide a progressive alternative to Feinstein — who does not support current single-payer Medicare for All legislation and who has voted to confirm half of Trump’s 22 highest-level appointees — having two progressives might be too much of a good thing.

They could split the left-leaning vote in the primary, leaving more centrist voters to Feinstein and possibly even allowing a Republican to sneak into the top two finalists to advance to the general election in November 2018. That possibility remains remote, at least for now, since no well-funded Republican with any kind of name recognition is in the race.

“I don’t know if there is enough (room for two progressives). I think that’s a problem,” said RoseAnn DeMoro, executive director of the 190,000-member National Nurses United and an influential voice in progressive state political circles. She praised both men as being more progressive than Feinstein and encouraged them both to “jump in with both feet.”

Still, she acknowledged that either would face an uphill fight.

Feinstein has “got the entire political establishment with her. She’s got the money (a net worth of $79 million). She’s got the standing. She’s got the office,” DeMoro said. “She’s going to be a formidable candidate.”

De León and Steyer were described as political “soul mates” over their bond over climate change and other progressive issues in a 2015 CalMatters story. “We share the same goals together,” de León told the publication.

While Steyer and de León may be political soul mates, they will be likely to appeal to different constituencies. Steyer was born in New York, the Ivy League-educated son of an attorney. He amassed his estimated $1.6 billion personal wealth by running a hedge fund in San Francisco for 26 years. For the past several years, he has become one of the Democratic Party’s top funders, focusing on environmental issues and improving voter registration and turnout, particularly among Millennial voters.

De León was born in San Diego, the son of a maid who emigrated from Mexico. He didn’t graduate from college until he was 36 and rose into the upper levels of state power as a grassroots community organizer. Analysts say he could tap into a coalition of progressive voters — Latinos, African Americans and Millennials — who don’t often vote in midterm elections, but might this year when there will likely be high-profile congressional elections in Orange County and the Central Valley. There also could be a person of color — state Treasurer John Chiang or former Los Angeles Mayor Antonio Villaraigosa — on the gubernatorial ballot.

Neither man, however, has much name recognition outside of political circles.

“Money and connections” are the differences between them, said Louis DeSipio, a professor of political science and Chicano/Latino Studies at UC Irvine. “De León has connections inside the state, but not a broad network outside the state. He would be at a financial disadvantage.”

And, DeSipio said, it would take a lot of money to organize a get-out-the-vote program for young and Latino voters.

Still, some analysts said de León could paint a broader contrast to Feinstein, 84, who is seeking her sixth term.

“Kevin clearly can point to his success as a state legislator, and being a Latino would appeal to the increasing number of Latino voters in California,” said Darry Sragow, publisher of the nonpartisan California Target Book and a longtime Democratic strategist who managed Feinstein’s first statewide campaign in 1990.

Being from Los Angeles, where more than 40 percent of the voters live helps, too. “You’d have to give it to Kevin de León on points there,” Sragow said. “The question is: Does he have any hope of raising enough money to run a competitive campaign?”

What the two men agree on is that Feinstein has not been tough enough on Trump. In a letter this week to Rep. Ben Ray Luján, D-N.M., chair of the Democratic Congressional Campaign Committee, Steyer demanded that Luján make public his position on whether Trump should be impeached.

But the subtext of the letter was a thinly veiled slap at Feinstein, alluding to her comments last month at San Francisco’s Commonwealth Club, where she said she hoped that Trump “has the ability to learn and to change. And if he does, he can be a good president. And that’s my hope.”

For Aram Fischer, a leader of Indivisible San Francisco, which at 4,500 members is one of the largest local resistance groups in the nation, the good news is that if one or both progressives challenge Feinstein, “there’s going to be a genuine exchange of ideas. We’re excited about it.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli