Barbara A. Mikulski (D) beat back a well-financed challenger to win her fourth term in the U.S. Senate yesterday, as Democrat John F. Kerry easily won Maryland's 10 electoral votes in the presidential race.

Mikulski, in claiming victory, declared that Maryland remains solidly in the Democratic column, despite the Republican now in the governor's mansion.

"Maryland has come back to the Democrats," she said. "We are a blue state, we are neon blue, we are cobalt blue, we are blue in the face."

But Republican leaders, pointing to a margin of victory smaller than in Mikulski's past campaigns, suggested that the GOP is gaining ground in the heavily Democratic state.

Mikulski won about two-thirds of the vote, a slightly smaller showing than her previous reelection bids. She won those contests, in 1992 and 1998, with 71 percent of the vote each time. She initially won the Senate seat with 60 percent of the vote in 1986.

Her challenger, Republican state Sen. E.J. Pipkin (Queen Anne's), vowed to return. "As the governor of California said, 'I'll be back,' " Pipkin told a boisterous crowd gathered at an Annapolis hotel.

Pipkin poured more than $1 million of his own money into an energetic bid to unseat a liberal icon and a politician whose long tenure has made her a familiar face across the state. The race was a test of the Republican assertion that Maryland is becoming more politically conservative. The 2002 election of Gov. Robert L. Ehrlich Jr., Maryland's first GOP governor in more than three decades, appeared to inaugurate what the Republicans called a sea change, and Pipkin's showing yesterday may bolster their argument.

"Tonight is a celebration of the future," Ehrlich told the crowd after Pipkin conceded. "This is about communicating a message to a growing Republican base."

As expected, Mikulski fared best in her native Baltimore and the Washington suburbs. Pipkin won some counties in western and northern Maryland, as well as the Eastern Shore county where he lives. Other Eastern Shore communities, though, voted for the incumbent.

Pipkin, 48, a better-funded opponent than most of Mikulski's previous challengers, never seemed to draw close to the three-term incumbent. A poll conducted early last month placed him 24 points behind Mikulski, but a sounding last week by a different research firm showed him lagging by 37 points.

His advantage over previous GOP challengers was his wallet: He had spent $1.7 million by mid-October, more than $1 million of which was his own money. Mikulski had spent more than $4 million at that point, out of a campaign fund of nearly $5.6 million, making this race one of the most expensive in Maryland history.

Echoing Mikulski's own appeal to voters, Pipkin emphasized his working-class roots. He also reminded voters of the issue that brought him into politics -- a late 1990s plan to dump millions of cubic yards of dredge spoil cleared from shipping channels into the Chesapeake Bay. Pipkin's grass-roots campaign helped kill the plan and propel him to the General Assembly.

After a successful career on Wall Street, where he made a multimillion-dollar fortune as a bond trader, he returned to live in Maryland in 1999. He defeated an entrenched Democrat to win a state Senate seat in 2002.

In campaigning against Mikulski, he used a series of sometimes light-hearted television advertisements to remind voters that the senator has voted to raise taxes, by his count, 350 times, and questioned her support for national security and the environment. "Who knew?" the ads asked, implying that Mikulski is not what she seems.

She pushed back on all counts. Although never denying that she has supported some tax increases, Mikulski said Pipkin's number was a distortion of her record -- a "cookie-cutter" tactic out of what she called the "right-wing playbook."

She defended her 2002 vote not to authorize military action in Iraq by saying she favored a more direct focus on the "war on terrorism" and also noted that environmental groups have regularly applauded her voting record in the Senate.

Mikulski, 68, has been in Congress for 28 years -- 10 in the House and 18 in the Senate -- and she has consistently presented herself as a "fighter for Maryland," an image she promoted again in this campaign. She reminded Marylanders of the federal largesse she has helped to send their way, such as funding to rebuild urban neighborhoods and support for fire departments, and emphasized, to Democrats, the need to strive for the party to retake the Senate.

Mikulski sometimes referred dismissively to her opponent. "We don't want to hear a peep out of Pipkin," she said. Toward the end of the race, she noted his background in finance, calling him a "risky investment."

In the eyes of some Democrats, Pipkin posed a greater challenge than Mikulski's opponents in 1998, when she faced a perennial candidate and retired surgeon named Ross Z. Pierpont, and 1992, when Alan L. Keyes ran against her.

Sen. Barbara A. Mikulski celebrates at the Wyndham Hotel in Baltimore. "We are a blue state," she told supporters. "We are neon blue, we are cobalt blue, we are blue in the face."