Democrat Niki Tsongas cast her first House vote Thursday on an issue that helped propel her into Congress: expanding a children's health insurance program.

Shortly after being sworn in to the seat her late husband Paul Tsongas held in the 1970s, she joined her Massachusetts colleagues in voting to override President Bush's veto of a bill that would have expanded the State Children's Health Insurance Program. The effort failed by 13 votes.

Democrats had pushed for the vote after Bush vetoed legislation to boost spending for the program known as SCHIP by $35 billion over five years. Bush wants a $5 billion increase.

Tsongas made her support of the program a central issue in the closing weeks of her campaign against Republican Jim Ogonowski in the 5th Congressional District. She said the issue was "tremendously important" to her victory in Tuesday's special election.

"It's an important generational issue," Tsongas said. "One thing I've learned during the campaign is that if you don't have access to health care when you're young, it complicates life as you go through adulthood."

Earlier in the day, Tsongas, flanked by other members of the state's Congressional delegation, was sworn on the House floor by House Speaker Nancy Pelosi of California.

"It's a real honor and pleasure and treasure to be sworn in by the first female House speaker," Tsongas said.

Massachusetts hasn't been represented by a woman since 1983, when Republican Margaret Heckler left office after losing a re-election bid. Pelosi said Tsongas' election was long overdue.

"We're thrilled to death to have another woman in the Congress of the United States," Pelosi said.

Pelosi signaled that Tsongas would win a seat she wanted on the Armed Services Committee. Meehan served on the panel, which is important to firms in the Merrimack Valley district that do defense-related work.

Tsongas' first official day in Washington resembled a victory lap, filled with hearty handshakes and a congratulatory words. On the House floor, Rep. Barney Frank greeted her with a pat on the back and a big hug. Rep. James McGovern kissed her on the cheek.

She began making the rounds on Capitol Hill early with a morning meeting in the offices previously occupied by the man she succeeded, Lowell Democrat Martin Meehan. Meehan resigned in July to become the chancellor of the University of Massachusetts-Lowell.

The beige walls of her new offices were bare and the desks empty, but Tsongas, who was accompanied by her three daughters and a throng of friends and former congressional staffers for her late husband, seemed impressed.

"It's rather grand," she said of the high-ceilinged rooms with plush blue carpeting.

Tsongas quickly noted the spacious offices could be short-lived, since she's only inheriting Meehan's offices temporarily. If she wins re-election in 2008, she will likely have to move to smaller offices due to her low standing on the House seniority ladder.

The visit even sparked a brief flashback to the days when she was on Capitol Hill with her late husband. Tsongas, 61, recalled being in her late husband's congressional offices three decades ago with her daughter Ashley, who was then a year old. Ashley is now 33.

"It tells you how life evolves," she said.