WASHINGTON - Congress returns to Washington today with a full slate of must-do legislation, just three weeks before the Christmas recess and with four members of the slim Democratic Senate majority likely to miss votes as they campaign for president.

The lawmakers' to-do list would be daunting under the best of circumstances: a major energy bill, legislation to rein in President Bush's warrantless wiretapping program, 11 of the 12 annual bills to fund the government, a farm bill, and a bill to stave off expansion of the alternative minimum tax and extend a raft of expiring tax credits.

But Senate Majority Leader Harry Reid (D., Nev.) must tackle that agenda and battle a combative GOP minority and an intransigent Republican president without a reliable majority.

"The majority leader's job is always tough, and his job has been made all the more difficult by the presidential candidates," said Senate Minority Whip Trent Lott (R., Miss.), who will resign by month's end. "But if you're going to run for president, you've got to get out there and run for president."

Senate Democrats normally can count on a 51-49 majority, assuming independent Sen. Joseph Lieberman of Connecticut sides with his old party. With Sens. Hillary Rodham Clinton (D., N.Y.), Barack Obama (D., Ill.), Joseph R. Biden Jr. (D., Del.), Christopher Dodd (D., Conn.) and John McCain (R., Ariz.) campaigning furiously for their parties' presidential nominations, Republicans can have an effective 48-47 majority, with an extra vote from Lieberman on most national security issues.

Many of those candidates have warned their leadership not to expect them around much with the Iowa caucuses just a month away. Senate Majority Whip Richard J. Durbin (D., Ill.) said he did not expect to see the candidates during Senate debates but hoped to schedule votes that allow them time to return to Washington.

The alternative minimum tax (AMT) bill is already dangerously overdue. Without it, a parallel tax system enacted in 1969 to ensure that 155 super-wealthy families would pay at least some income taxes would reach as many as 23 million more families this year, mainly those with upper-middle incomes.

But Democrats have been stymied by their own pledge to pay for the AMT fix and any other measure that increases the deficit with offsetting tax increases or spending cuts. Those offsets are running into opposition in the Senate from Republicans and some Democrats.

The IRS's independent oversight board last week voiced "grave concerns about the serious risks to the 2008 filing season" posed by Congress' delay on the AMT.

But the tax bill is not even on the Senate's calendar for this week. Instead, Reid would like to wrap up work on the farm bill, then tackle warrantless wiretapping.

The House this week hopes to finish an energy bill that would raise auto fuel-economy standards for the first time in 32 years, then hand it off to the Senate for final passage the following week.

With the presidential campaign in full swing, how the Senate will handle any moderately controversial legislation is a mystery - even to Democratic leaders.

"As the Democratic whip, I'm making a list and checking it twice," Durbin said.

In the Senate, where schedules are famously unreliable, leaders must jump at any opportunity to hold final votes. On Nov. 8, at 11 p.m., when a window opened suddenly to confirm Michael B. Mukasey as attorney general, not one of the five presidential candidates was on hand to vote.

Earlier that day, all five missed the first veto override of the Bush presidency, when Congress salvaged a water-projects law. And only Biden was on hand the week before for final passage of a bill expanding the State Children's Health Insurance Program.

Durbin noted that the Democratic candidates hopped a red-eye flight after their Nov. 15 debate in Nevada to make a 9 a.m. vote on Iraq war funding. Clinton then turned around and flew to the West Coast.