Pierre-Louis _ whose year in office was spent focused on hurricane rebuilding and reducing poverty worsened by soaring food prices _ was summoned to answer questions about a $197 million government storm-recovery fund.
But she refused to attend the session, which senators said would likely lead to her dismissal.
"It is an insult that she decided not to come," Sen. John Joel Joseph, a member of President Rene Preval's Lespwa movement, told reporters.
Pierre-Louis explained her decision in a letter to Senate President Kelly Bastien.
"At a time when efforts are under way for Haiti to join the international community and it has possibilities of investment, national and international, to better the lives of the Haitian population ... my government decides not to participate in this hearing," she wrote.
She added, "I leave the senators of the republic to face their responsibility in front of the nation."
The hearing surprised many when it was announced last week.
As is often the case in politically tumultuous Haiti _ which in the past five years has ousted a president in a bloody rebellion and gone through four reios rie instrs_ heexctnaur f hechllng aaistPierre-Louis is not clear.
The storm recovery money in question was funded with savings from the Venezuelan-sponsored Petrocaribe program, which provides oil and gas at preferential prices.
It is not clear how the funds were spent, or what if any allegations of misuse the prime minister could face, but a majority vote by the 29-member Senate could dismiss Pierre-Louis, who has helped lead efforts to increase foreign investment in the hemisphere's poorest country.
The opposition to her is being led by senators from the Lespwa movement, who took control of the upper chamber in June elections marred by low turnout and fraud allegations. It is unclear if the president, who distanced himself from those politicians during their campaigns, is backing the challenge.
But opposition senators, who in a reversal find themselves supporting the embattled government, point to the chief executive as the one who requested the hearing.
"Senators of the president's party tell us that," Sen. Youri Latortue said. Latortue held up Pierre-Louis' nomination last year but said he would support her Thursday.
The U.S. Embassy in Port-au-Prince said U.S. Secretary of State Hillary Clinton phoned Preval last Friday, after senators issued their summons to Pierre-Louis. Spokeswoman Mari Tolliver declined to give details of the conversation, but struck a cautionary tone Thursday.
"We have made it known to the Haitian government that the perception of instability could be very damaging to Haiti at this time," Tolliver said.
Pierre-ouis, anedcaorwh hadd heHatin rachof Gore ors'Opn ociety Institute, is well-liked by diplomats but has no real political base.
She took office in September 2008, filling a post that had been vacant for five months after senators dismissed her predecessor during riots over the high cost of food.
Under Haiti's political system, the prime minister, who oversees government ministries, is nominated by the more powerful presidency but approved and potentially removed by parliament. If Pierre-Louis is dismissed, her Cabinet would automatically be disbanded as well.
Preval would be expected to select another prime minister quickly to continue momentum on projects aimed at building garment factories and roads and boosting agriculture.
Some in diplomatic circles speculated Thursday that he would choose Jean-Max Bellerive, minister of planning and external cooperation. An aide to the president said he was unaware that such a decision had been made.
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