Rep. Chris Van Hollen has raised more than $1 million for what could be a bruising and expensive primary campaign to succeed retiring Sen. Barbara A. Mikulski.

From January to April, Van Hollen (D-Md.) took in upwards of $1.15 million, his campaign said Wednesday. The vast majority of the funds were donated in the four weeks since Mikulski (D-Md.) announced that she will step down in 2016.

Van Hollen launched his campaign to succeed the veteran lawmaker days later.

He will face at least one high-profile challenger in the Democratic primary: fellow Rep. Donna F. Edwards of Prince George’s County.

Van Hollen — the ranking Democrat on the Budget Committee, former chairman of the Democratic Congressional Campaign Committee and a representative of well-off Montgomery County — brings considerable fundraising skills to the race.

In last year’s election, Van Hollen generated about $1.5 million for House Democrats and candidates, making him one of the caucus’s best money-raisers. He was tapped to serve as the DCCC’s finance chairman this cycle, but because of Van Hollen’s Senate campaign, Rep. Don Beyer (D-Va.) has taken over that role.

More than three-quarters of Van Hollen’s haul during the first quarter of 2015 came from donors based in Maryland, according to the campaign. He  now has $2.5 million on hand.

Edwards has not released her first-quarter numbers. The congresswoman is not a prodigious fundraiser, a limitation that likely cost her a chance at House leadership and one she will have to overcome to compete in the Senate primary.

She will have help, however. Emily’s List, which spent more than $40 million in the 2014 elections, is backing Edwards, as are Democracy for America and the Progressive Change Campaign Committee.

Sen. Kirsten Gillibrand (D-N.Y.), a powerful fundraiser with Wall Street connections, is also supporting Edwards. In an early March fundraising e-mail, Kimberly Oxholm of the Women Donors Network said that Edwards needed to raise $1 million by the end of the month.

People involved with Maryland politics say they expect others to join the Senate race, the state’s first for an open seat since 2006. Reps. Elijah E. Cummings, John Delaney, C.A. Dutch Ruppersberger and John P. Sarbanes are all potential Democratic primary candidates.

Of those, the Baltimore-based Cummings is widely seen as the most likely and most formidable. He has already polled the race, and the numbers were very encouraging, according to a Democratic strategist familiar with the results.

Former NAACP president Benjamin Jealous, former lieutenant governor Kathleen Kennedy Townsend and former state delegate Heather R. Mizeur are considering Democratic primary bids as well. Baltimore Mayor Stephanie Rawlings-Blake (D) has not officially declared herself out of the race, but she has said she would step aside for another Baltimore candidate — Cummings, in particular.

On the Republican side, former Senate candidate Dan Bongino; former governor Robert L. Ehrlich Jr.; Ehrlich’s wife, Kendel; U.S. Rep. Andy Harris; and former lieutenant governor Michael Steele have been mentioned as people who could enter the race.

Detailed fundraising reports for the first four months of the year are due for all candidates on April 15.