The absence of Senate Appropriations Committee Chairman Robert Byrd (D-W.Va.) is forcing Democrats to make contingency plans as budget season moves into high gear.

Since the iconic 90-year-old senator was first hospitalized two weeks ago, Democrats have filled in twice to chair hearings he would normally preside over. They also approved a Senate rule allowing him to vote by proxy during a committee markup of a budget resolution and are now scrambling to make sure they have enough votes in the full Senate to push through a budget that has split along partisan lines.

The votes this week are expected to be so close that Senate Democratic leaders have asked their two presidential candidates, Sens. Barack Obama (Ill.) and Hillary Rodham Clinton (N.Y.), to take a break from a heavy campaign schedule to vote on Thursday.

If the outcome of an amendment or passage of the budget this week hinges on Byrd’s vote, Democrats might seek to reach an agreement with another senator to withhold his or her vote to balance out Byrd’s absence. It’s unclear whether that method, known as vote pairing, will be used and Democrats said Tuesday they had not yet reached out to Republicans to see if they would support such an approach.

A longer-term absence could make it tricky to sort out the appropriate amount of funding to be allocated for Appropriations subcommittees, a situation in which the panels’ chairmen plays a critical role. That could become even more complicated if Byrd is still absent and Democrats fail to win approval of their budget resolution, which creates a blueprint for federal spending in fiscal 2009.

The decisions on subcommittee allocations likely won’t be finalized until mid-to-late April.

“It may cause some difficulty in resolving the allocations,” said an appropriations lobbyist, who asked for anonymity in discussing the implications of Byrd’s health. “Someone has to make the final call. If Chairman Byrd can’t be as actively involved as he has in the past, it will take longer.”

But, according to some aides, his presence is already being missed since the chairman is influential in fending off amendments to the budget that could hamstring the Appropriations Committee.

“His absence is felt,” said a senior Senate aide who also requested anonymity. The aide added that a budget conference without the Appropriations chairman could put the Senate in a tough negotiating position.

Jesse Jacobs, a spokesman for the senator, said Byrd is “frustrated” that he is not back on Capitol Hill.

Jacobs said that Byrd is continuing to enroll bills in his capacity as president pro tempore of the Senate and “remains involved” in the appropriations process by having discussions with staff and members. Jacobs said the senator might even be back for votes Thursday, when the chamber is expected to consider a slew of amendments to the budget resolution.

Byrd, who is the longest-serving senator in U.S. history and is third in the line of presidential succession, was first hospitalized on Feb. 26 after suffering a fall in his Virginia home and injuring his back. He was discharged after staying at Walter Reed for three nights, and his office expected him to return to work the following week.

But Byrd suffered a urinary tract infection and his doctors advised him to stay at home until the infection had run its course. On March 5, Byrd was readmitted to a local hospital after doctors expressed concerns over the senator’s reaction to antibiotics he had been taking.

“According to his doctors, Byrd’s antibiotics are causing a reaction and some additional tests are needed in order to change the antibiotic medicine to one that the senator can better tolerate in the continuing treatment of a urinary tract infection. His stay is expected to be brief,” his office said when he was readmitted.

Most Democrats on the Appropriations Committee do not think the appropriations process has been overly affected because of Byrd’s absence, since budget season is just starting and will get into full swing by the spring or summer.

“Everybody personally misses Sen. Byrd because he’s such an able chairman and has so much experience,” said Sen. Ben Nelson (D-Neb.), an appropriator. “But there are others who have been around an awful long time who I think are always able to step up at a moment’s notice or less and be helpful.”

One of those senators is Patrick Leahy (D-Vt.), who filled in to chair an Appropriations Committee hearing Tuesday on “waste, fraud and abuse” in Iraq. Byrd called Leahy yesterday to ask him to preside over the hearing.

Leahy’s name has been floated as a possible successor to Byrd, but he would have to relinquish his control of the Judiciary Committee.

When asked if he was seeking the chairmanship of Appropriations, Leahy said on Tuesday: “I love being chairman of Judiciary.”

The next in line to succeed Byrd is Sen. Daniel Inouye (D-Hawaii), who presided over a hearing last week held by Byrd’s Homeland Security Appropriations subcommittee, but couldn’t preside over Tuesday’s hearing because of a scheduling conflict.

Inouye, chairman of the powerful Appropriations Defense Subcommittee, said he didn’t even “want to think about” succeeding Byrd, but noted: “I’m next in line.”

Sen. Patty Murray (D-Wash.) has also been mentioned as a possible successor to Byrd, but she has dismissed such talk in the past.

Lawmakers said they have to find a way to get through their heavy workload even if senior members may be out from time to time with illnesses, as was the case with the recent hospitalization of House Ways and Means Committee Chairman Charles Rangel (D-N.Y.).

 “We will keep working,” Leahy said. “The Senate always tends to work things out.”