WASHINGTON (AP) - Mad cow disease and election-year pressures are complicating White House and Republican leadership efforts to push a long-overdue $373 billion spending bill through Congress.

The Senate was poised for a pivotal vote Tuesday, the start of the second session of the 108th Congress, on whether to end Democratic delays that have thwarted the bill's passage. GOP leaders need 60 of the Senate's 100 votes to end the roadblocks, a margin both sides say will be tough to attain.

Eager to get the long-running issue off the Senate's plate, top Republicans were pointedly reminding colleagues that the bill, which combines seven spending measures into one, contained thousands of home-state projects.

Republican leaders also were threatening that failure would mean a pared-down version would take the bill's place - $6 billion smaller and shorn of lawmakers' projects and increases for popular programs.

''The time has come to pass this bill,'' Senate Majority Leader Bill Frist, R-Tenn., wrote senators last week.

But, if anything, the odds of Senate passage have grown bleaker since the House approved the measure in December. The discovery of a Holstein cow with mad cow disease in Washington state has intensified anger over a provision in the bill that would delay country-of-origin labels on foods.