Chris Murphy, who leads the U.S. Senate subcommittee on relations with the Middle East, said "I think we've got a little bit of work to do to convince all of our European friends that this is the time to drive the harder bargain."
He said that European nations were likely looking at "short-term stability" but called for a longer-term view.
Tunisia reached a deal in principle in October with the International Monetary Fund for nearly $2 billion to shore up a sinking economy, but the bailout still needs approval by the IMF board which is pressing for reforms, primarily on the economy.
President Saied vowed last week to reject IMF "diktats" and Italy has led calls for a quick deal, fearing a greater flow of migrants across the Mediterranean if Tunisia descends into economic or political chaos.
Murphy said "Yes, the IMF could step in here with a deal with few conditions that would patch things up for months, maybe years.
"But you'd be back in a crisis - once again you'd be necessitating another bailout, that would be even less palatable to international financial institutions - a few years from now."
Murphy also voiced opposition to Biden's budget proposal that would keep funding levels for the Tunisian military, which the senator said has "largely stood by, acquiesced and sometimes facilitated Saied's transition away from democracy."
"The Biden administration has, I think, made a bet on the Tunisian military," Murphy said. "I would argue that we should make a bet on civil society instead."
Saied has seized far-reaching powers since sacking the government in July 2021, later dissolving parliament and pushing through a constitution replacing the one approved in 2014 when Tunisia was at the forefront of the Arab Spring democratic revolutions.