Faced with a difficult choice in the Capitol and a difficult election in Louisiana, Democratic Sen. Mary Landrieu stood in the well of the Senate until time to vote had nearly expired.

Finally, with the issue already settled, she cast her vote with President Bush and the Republicans, and against a last-minute Democratic attempt to change legislation creating a new Department of Homeland Security. It was important, she said afterward, "for me to be able to express that I'm for the creation of this department and I've been for the creation of this department.''

Other senators were unusually blunt in describing the political forces at work. 

"I think she did the right thing,'' said Louisiana's other Democratic senator, John Breaux, who supported the proposal that Landrieu opposed. If Landrieu had voted the other way, "It would have created a new issue for a whole new set of ads'' for Republicans to air in her runoff election on Dec. 7, he added.

The chairman of the GOP senatorial campaign committee seemed to agree. "If she had voted to obstruct the president's leadership it would have hurt her in Louisiana,'' said Sen. Bill Frist, R-Tenn. "So from a political standpoint, it was a wise decision on her part.''

Landrieu, a 47-year-old first-term senator, was in a unique position as the roll was called, the result of her state's open primary system. While 49 states were electing members of Congress on Nov. 5, Louisiana held an open primary.

Landrieu easily led the field with 46 percent of the vote. Short of a majority, she was pushed into a runoff on Dec. 7 against Republican Suzanne Haik Terrell, the state commissioner of elections. Terrell won only 27 percent of the vote, but the Republican vote combined was 51 percent, and polling suggests a competitive run-off.

Both political parties have plunged into the Louisiana race. Republicans are eager to build on the midterm election triumphs that delivered them control of both houses of Congress, and Democrats are hoping to prevent the loss of a seat that would push them further into the Senate minority.

Vice President Dick Cheney raised money for Terrell in Louisiana on Monday. GOP officials said they were arranging a campaign trip by New York Mayor Rudolph Guiliani. Bush, too, will be campaigning for Terrell, according to GOP sources who spoke on condition of anonymity, with stops likely in two or three cities.

The homeland security legislation has been the focus of a pitched battle in the Senate, and much of the maneuvering has concerned civil service rules and job protections for employees of the proposed new department. Democrats, aligned with labor unions, accused Bush of seeking to overturn years of job protections. Republicans said their legislation merely gave Bush the same authority that other recent presidents have had.

Bush routinely raised the issue in his campaign appearances, saying that the Democratic-controlled Senate was "more interested in special interests'' than in the national interest.

The issue figured prominently, as well, in Democratic Sen. Max Cleland's defeat in Georgia this fall. Cleland backed legislation to create a Homeland Security Department even before Bush embraced the idea. But he sided with Democrats on the issue of worker protections, and in a commercial in the campaign's final weeks, Rep. Saxby Chambliss accused him of voting "against the president's vital homeland security efforts 11 times.''

For her part, Landrieu has long made clear her support for homeland security legislation. At the same time, she sometimes voted with Democrats on a series of procedural votes earlier in the debate. Some Republicans, speaking on condition of anonymity, grumbled that debate on the legislation during the lame-duck session of Congress would give her an opportunity to blunt any campaign attacks on the issue.

"It's been made much too political,'' Landrieu said of the debate over the legislation. "It's very unfortunate that this situation has been politicized because there's a lot at stake.''

She stood in the front of the Senate as the roll was called. The administration's victory was sealed when Maine GOP Sens. Olympia Snowe and Susan Collins voted against the Democratic proposal for last-minute changes in the measure.

Only then did Landrieu cast her vote. She was one of only three Democrats to side with the White House.