Mfume (D), a former congressman from Baltimore, announced his candidacy Monday, and Rep. Albert R. Wynn (D-Dist. 4) of Mitchellville dropped out, deferring to Mfume.

Van Hollen (D-Dist. 8), meanwhile, said he would consider a run seriously, but needs to talk it over with his family and his supporters. That has the potential to create its own scramble in congressional District 8, which is mostly in Montgomery but includes a sliver of Prince George's County.

Baltimore-area Reps. Benjamin L. Cardin (Dist. 3) and C.A. "Dutch" Ruppersberger (Dist. 2) are also weighing runs on the Democratic side, with Republican speculation centering on Lt. Gov. Michael S. Steele. "Every congressman from Maryland should be looking at this seat," said Patrick Gonzales, an Annapolis pollster. "You're going to see a highly contested Democratic primary and some action from the Republican side. It's going to be a wild and wooly affair."

A Gonzales poll released in January on a potential Senate primary had Baltimore Mayor Martin O'Malley (D) leading the pack when matched up with Mfume and Cardin. O'Malley led with 26 percent to Mfume's 24 percent to Cardin's 20 percent.

But O'Malley and Montgomery County Executive Douglas M. Duncan (D) have said they are not interested in running for the Senate, despite the hopes from some Democrats of avoiding a bloody primary in next year's gubernatorial race.

"My gut reaction is [Sarbanes' retirement] opens up the possibility that Democrats will not have a contested primary for governor, which would be very important," Senate President Thomas V. Mike Miller Jr. (D-Dist. 27) of Chesapeake Beach said.

If Van Hollen runs for Senate, the host of candidates who might seek to replace him begin with the two men Van Hollen defeated in a vigorous 2002 primary, former Del. Mark Kennedy Shriver and former trade ambassador Ira S. Shapiro. Other Democrats who could be interested include Susan Turnbull, vice chairwoman of the Democratic National Committee; Maryland Democratic Party Chairman Terry L. Lierman; state Sens. Brian E. Frosh (Dist. 16) and Robert J. Garagiola (Dist. 15); and members of the County Council, including President Thomas E. Perez and Philip M. Andrews.

On the Republican side, Steele, who has been building a national reputation in the GOP, has expressed interest, but he would have to decide whether to split from Gov. Robert L. Ehrlich Jr. as he gears up for re-election. Steele has said he wants to run for governor in 2010, predicting that he and Ehrlich will win re-election in 2006. Del. Jean B. Cryor (R-Dist. 15) of Potomac also may be interested.