They also had another concern: real estate. The Democratic Senatorial Campaign Committee is millions of dollars in debt, not only from the hard-fought midterm campaign, but also from the cost of a $5.2 million mortgage taken out to buy additional office space last spring.

In what appears to be an effort to help meet such costs, Democrats demanded that the added provision also allow all party committees, including the House and Senate campaign committees, to solicit additional large contributions to cover the costs of buying, renting or renovating real estate. The Democrats’ campaign committee and its leaders were not consulted about the request, a spokesman said.

“The D.S.C.C. was not involved,” said Justin Barasky, a spokesman for the committee.

Democrats also asked for a provision that would allow triple the normal amount for contributions earmarked for legal costs. Republican efforts to place new restrictions on voting in many states, two Democrats said, had prompted a surge in election litigation. The additional provision would allow donors to write a third check, again at triple the normal limit, to “to defray expenses incurred with respect to the preparation for and the conduct of election recounts and contests and other legal proceedings,” according to the bill.

In effect, the new rules whittle away at the restrictions on so-called soft money in the 2002 McCain-Feingold legislation, which prohibited parties from raising large contributions under the guise of funding “party-building” and get-out-the-vote efforts.

All in all, the new accounts would vastly expand the amount that wealthy donors could give to party committees. Under current rules, the most one individual could donate to party committees in a given year totals about $97,200. Should the new budget agreement be signed by President Obama, that amount would skyrocket to $776,600, or $1.56 million over a two-year election cycle.

In a letter to Mr. Obama on Friday, advocates of stricter campaign limits said the proposed changes amounted to “the most corrupting campaign finance provisions ever enacted” and urged him to veto the bill. “In a ‘bipartisan’ unholy alliance, Senator Reid and Senator McConnell joined with House Speaker John Boehner to secretly insert into the Omnibus bill the destructive campaign finance provisions, which were unknown to the public and members of Congress until the day the bill was filed in the House,” the letter stated.