A group of at least 13 Democratic senators, led by Jay Rockefeller of West Virginia and Tom Harkin of Iowa plan to send a letter to President Barack Obama laying out principles for a budget deal.

The letter calls on Obama to strike a deal on the fiscal cliff that has a 1-to-1 ratio of tax increases to spending cuts.

“These revenues must be real and not inflated by ‘fuzzy math’ like dynamic scoring,” the letter reads. “Any deal should end the Bush tax cuts for the wealthiest two percent of the population and close tax loopholes benefitting wealthy Americans and corporations. Furthermore any deal must include a one-to-one ratio of revenues to spending cuts.”

The draft of the letter seen by Roll Call did not include the names of the other signatories.

The letter lays down a marker for liberal senators as congressional leaders are set to meet with the president Friday. Congress must come to an agreement on a budget deal by the end of the year to avoid about $600 billion in tax increases and spending cuts that budget analysts say could throw the economy back into recession.

The signatories also want to count the $917 billion in already-agreed-to cuts included in the deal to raise the debt ceiling in August 2011 as spending cuts in any new budget deal.

“To ignore the significance of these cuts — by not counting them — further threatens programs that benefit working families,” the letter said.

The letter also calls for a deal to include a further extension of unemployment benefits and more spending to spur job creation in the short term.

Changes to Medicaid, Medicare and Social Security should also be rejected, the letter said.

“Each of these programs is a vital lifeline to the middle class,” the letter said.

The senators also stressed that the process by which any deal is approved should be transparent.

“Members should not be forced to vote up or down on plan that they have not seen and have not been able to study,” the letter said. “We should have ample opportunity to comment on proposals being considered and all viewpoints from the liberal to conservative end of the spectrum should be considered.”