DOVER — The Democratic primary for the U.S. House of Representatives is shaping up to be a major heavyweight fight.

With each of the four official candidates raising in excess of $81,000 in 2015, three of the four reported having at least $121,000 on hand, according to documents recently filed with the Federal Election Commission.

Each of the four Democrats has government experience and three have run for office before.

On the Republican side, a clear front-runner has emerged in the two-person primary, with a wide disparity in fundraising.

The incumbent, U.S. Rep. John Carney, a Democrat, is running for governor. That means Delaware’s lone House seat is open for only the second time since 1992.

For Democrats, Newark Sen. Bryan Townsend leads the way, raising $212,676 since declaring his candidacy Sept. 17. In second place is Rep. Bryon Short, of the Arden area, who entered the race one week later and brought in $152,253.

Former state Labor Secretary Lisa Blunt Rochester raised $121,522 after declaring her intentions Oct. 27. The last to enter the race, 2014 state treasurer nominee Sean Barney, has brought in $81,149 since Dec. 1.

The first three are very close in cash on-hand, with Sen. Townsend reporting $123,872, Rep. Short declaring $121,014 and Ms. Blunt Rochester announcing $127,057. Mr. Barney has $78,011 available.

Much of Mr. Barney’s money came from out of state. The other three Democrats received most of their donations from Delawareans, including lobbyists and a handful of state officials.

According to the respective campaigns, 136 people contributed to Mr. Barney, 443 gave to Rep. Short, about 460 donated to Sen. Townsend and 317 funded Ms. Blunt Rochester.

Among Republicans, former Wyoming Mayor Hans Reigle, who declared his candidacy April 8, leads the way with $51,179 and $36,082 on hand. Rose Izzo, the 2014 nominee, lists raising just $17 and has $16. She filed Sept. 30.

Most of the contributions to Mr. Reigle came from Delaware residents, including several state officials.

Rep. Short, Ms. Blunt-Rochester and Mr. Barney have all worked for Delaware’s senior senator, Tom Carper, a Democrat.

Democratic candidate Mike Miller, who announced his intentions in October, is set to file within two weeks, according to his campaign. He previously ran unsuccessfully for the office in 2000 and 2008.

The primary is Sept. 13.