The proposed big increase in the level of funds for franked -- or free-postage -- mail allowed each senator next year and a move to allow Capitol Police to make arrests off Capitol grounds are expected to make for contentious debate today when the Senate takes up its fiscal 1991 legislative branch appropriations bill.

The bill totals $1.5 billion and contains $443 million to operate the Senate and other congressional agencies such as the Library of Congress, Government Printing Office and General Accounting Office. When the House operating budget of $667 million, which was approved Sunday, is included, Congress's legislative appropriations for next year would total $2.2 billion.

The Senate has led the way in making changes in use of franked mail by members, requiring public disclosure since 1985 and limits on each member's mass mailings since 1989. This year, however, the Senate Appropriations Committee chose to increase sharply the amount to be allocated to members from fiscal 1990's $24 million to $35 million in the year ending Sept. 30, 1991.

The higher amount is based on a new formula developed in the Senate Rules Committee and pushed by its chairman, Sen. Wendell H. Ford (D-Ky.), a candidate for Senate Democratic whip next year. It calls for members to be allocated an amount each year equal to the cost of one first-class mailing to each household in their states.

Sen. Don Nickles (R-Okla.), the ranking Republican on the Appropriations subcommittee on the legislative branch who has been a leader in seeking changes in the franking system, will propose a series of franking amendments, including one to cut back the amount to be divided among members, according to Senate sources. How deep a cut he will offer has not been decided, according to an aide.

Another Nickles amendment would attempt to end the practice of senators not up for election transferring their unused mail funds to those facing reelection who have used their allocations.

Nickles also has targeted a loophole that permits senators to carry over from one year to the next their unused mail allocations. Under that rule, a senator -- whose term lasts six years -- can save up mail funds until a year before his or her reelection and then spend it all. The House, in its new rules, requires members -- who are elected every two years -- to use their allocation within a year or lose it.

The Oklahoma senator will attempt to allow Senate carryovers only from one year to the next, and then only at a level of 50 percent of what was given originally for mass mailings, an aide said.

The Appropriations panel already has accepted another key Nickles change, which prohibited members from transferring funds to pay for franked mail from their official expense accounts or their political campaign funds. The House, in its new rules, will permit a member to supplement his mail account with up to $25,000 a year from his office accounts.

Ironically, the proposed Senate rules would permit senators to make transfers the other way, from their mail funds to their office accounts, while the House prohibits such moves.

More controversy is expected over language in the Senate bill that would increase the authority of Capitol Police force, which has come under criticism in recent years for attempting to expand the range of its activities.

The Senate committee has proposed expanding the basic law governing the force by adding to its powers to "police the United States Capitol builings and grounds" the following phrase: "and perform all other duties necessary thereto."

The Senate panel also would give Capitol Police officers for the first time authority to make arrests outside the Capitol grounds when a crime is committed in their presence while they are "performing official duties," and also when they do not witness the crime but have "reasonable grounds to believe that the person to be arrested has committed or is committing" a crime.

In justifying the changes, the Senate panel referred to a recent court case in which charges were dropped against individuals arrested by Capitol Police off Capitol grounds based on the word of witnesses to the crime.

A House Appropriations subcommittee, in its review of the Capitol Police this year, refused to include such language in its bill and voiced concern in its report that the force was attempting to seek additional vague powers.

The House panel also criticized expenditures for Capitol Police protective details, sent with traveling members of Congress, which it said were "rising at an excessive rate."

A spokesman for the Metropolitan Police Department yesterday said D.C. police had told Capitol Police officials it was concerned that the federal officers not undertake undercover or investigative operations off Capitol grounds unless they coordinate with D.C. police.