The Maryland lawmaker said he was concerned about the president's willingness to move to a "chained CPI" calculation of federal benefits, including Social Security, which would result in smaller annual cost-of-living increases for the recipients.
The Maryland lawmaker said that to win his approval, "it would be absolutely necessary to include a bump up to correct for that effect for people that were older, if you were to go down the road at all."
"I don't like it," he said, but added he was "reserving judgment on the final package."
Van Hollen also looked to squarely peg blame for the potential change on House Republicans, calling the concession "one of the demands Speaker [John] Boehner [(R-Ohio)] made."
He also blasted Boehner's so-called "Plan B" solution as "another way to provide benefits and relief for very high-income individuals" and vowed the Senate would not adopt the plan, slated for a vote as early as Thursday.
Under the Republican proposal, taxes would raise on all incomes over $1 million, but no other part of the "fiscal cliff" would be resolved. President Obama had originally asked for tax rates on all incomes above $250,000 to increase; his latest proposal revises that to affect taxpayers making over $400,000 per year.
"They're not going to adopt a Republican plan designed to again protect the very wealthy people again at the expense of the country," Van Hollen said.
Van Hollen said the Republican leadership was unable to strike a deal because of "extreme" elements in the party.
"Speaker Boehner's not able to get the very extreme Tea Party members of his caucus to go along, people who are way out of the mainstream are running the show and dictating the terms," Van Hollen said.