A bill advancing in the Senate that would require continued six-day delivery of mail and discourage the closing of postal facilities threatens to undermine a broader postal reform that already has passed the Senate, a main sponsor of that original legislation said Thursday.

Sen. Susan Collins (R-Maine), a sponsor of the original bill, and Sen. Richard J. Durbin (D-Ill.) squared off over the issue.

Collins said language in Thursday’s spending bill, sponsored by Durbin and approved on a party-line vote by the Senate Appropriations Committee, would put the original “comprehensive reform in jeopardy.”

Collins is the ranking Republican on the Homeland Security and Governmental Affairs Committee, which crafted the broader reform bill that passed the Senate with bipartisan support in April.

That bill would open the door to dropping a mail delivery day. It would not do so for at least two years after enactment, however, and not until the U.S. Postal Service had exercised the cost-cutting authorities in the reform plan and postal regulators had approved.

The bill also would not bar the closing of all of the hundreds of processing centers slated by USPS for shuttering, but rather would set certain delivery standards by region. The move effectively could protect more than 80 such facilities. In addition, the bill would impose several restrictions on when, where and how rural post offices could be closed.

Collins also is a member of the Appropriations Committee, which Thursday afternoon took up the financial services and general government spending bill.

That Durbin-sponsored bill would require that six-day delivery “shall continue without reduction,” that none of the money from the bill could be used to close small post offices in 2013, and that certain mail processing facilities — including two in Illinois — could not be closed before 2014 and not until after a study by the postal inspector general.

“I’m just very upset and disturbed,” Collins said. “We dealt with that in our bill in a more sophisticated way and a better way.”

She said the spending bill would effectively pick apart postal reform at a time when the House is preparing to take up the issue. House leaders have said they expect to consider their own plan to overhaul the cash-strapped USPS in July.

In a somewhat testy exchange with Collins, Durbin stressed that postal reform still is in process and discounted the notion that the spending bill would endanger that effort. He noted that the language on six-day deliveries and rural post offices was not new.

“There was nothing sinister to put this in here. It’s been in here many years,” he said.

Durbin added that he would “make no apologies” about the new provision regarding processing facilities, saying it would only require that facilities be given “a second look” to determine whether a closing is justified.

Collins ultimately did not offer an amendment to remove the postal provisions, meaning they will be part of the spending bill when the full Senate votes on it.