Nicaragua's opposition Liberal Party has condemned criticism by a US diplomat as "outrageous interference".
President Bolanos has warned of a slow-motion coup
Earlier this week, the US deputy secretary of state called former President Arnoldo Aleman - who has been convicted of fraud - "corrupt".
Robert Zoellick also threatened to cut off aid if Aleman continued to work with the leftist Sandinistas against President Enrique Bolanos.
Mr Zoellick and Mr Bolanos accuse the opposition of attempting a coup.
The Sandinistas have yet to respond to the comments by the US diplomat, who said at the end of a two-day visit that he felt the political ground was shifting.
The Liberals and Sandinistas dominate the Nicaraguan Congress, which is at present debating a proposal to impeach Mr Bolanos for alleged campaign finance violations.
The legislators have already stripped two of Bolanos' ministers and three senior officials of their immunity from prosecution.
The president, who took office in 2002 after a landslide victory over Sandinista leader Daniel Ortega, claims he is the innocent victim of an agreement between former president Aleman and Mr Ortega.
Members of Bolanos own Liberal Party turned against him - and joined forces with former rivals the Sandinistas - angered by the government's decision to prosecute Aleman for corruption.
Aleman is serving a 20-year sentence for fraud and money-laundering, but he still commands the loyalty of many of his party's legislators.