The two Democratic leaders of the State Legislature — Darrell Steinberg, the president pro tem of the Senate, and John A. Pérez, the speaker of the Assembly — announced on Wednesday that they would oppose the sale. Both men control seats on the boards of California’s major pension funds.

“Newspapers are public trusts, and I think it is wrong for The Los Angeles Times to end up in the hands of two people who have such a pronounced rigid ideology on a whole host of issues,” Mr. Steinberg said in an interview. Mr. Pérez, in a statement, said he was “deeply concerned about media outlets being purchased to further a political agenda.”

A liberal advocacy group, the Courage Campaign, bought advertisements to run in The Los Angeles Times on Thursday urging readers to cancel their subscriptions if the Tribune Company agrees to sell the newspaper to “the right-wing Koch Brothers.” More than 1,000 people have pledged to cancel their subscriptions, said Rick Jacobs, the head of the campaign, while 110,000 have signed petitions opposing the sale.

“The LAT has a long and storied past of publishers taking it in various directions,” Mr. Jacobs said in an e-mail. “The Kochs are much more likely to end journalism as we know it. They are likely to stop coverage of climate change or skew it. They will almost certainly change the way the LAT covers state politics, especially ballot measures.”

A red-on-black poster — “No Koch Hate in LA: Stop the Koch takeover of the L.A. Times” — went out on Wednesday urging people to turn out for a rally next Tuesday in front of Mr. Karsh’s office here.

“The Koch brothers would use the newspaper in an extremist and ideological way,” said María Elena Durazo, executive secretary-treasurer of the Los Angeles County Federation of Labor, A.F.L.-C.I.O. “And I’m saying that even though I believe the current L.A. Times is not particularly labor-friendly.”

Should the Koch brothers proceed with their efforts, and draw on their considerable financial assets to try to push rivals off the field, shareholders in the Tribune Company, including Oaktree and JPMorgan Chase & Company, might be hard-pressed to refuse a deal that would be seen as in the best financial interest of the company. The union leaders, in their letter to Oaktree, argued that the cost of selling to the Kochs, in the form of losing the investments of the pension funds, would be fiscally irresponsible.

“For those people whose retirement savings are invested in public pension funds, selling the Tribune Company’s newspapers to the Koch brothers so they could in turn use it to advance specific policies adverse to the retirement security of working people would be akin to agreeing to selling your car to a buyer who you know wants to buy the car so they can run you over,” the letter said.