DOVER — The race to become the next occupant of Delaware’s sole seat in the U.S. House of Representatives just got more crowded.

Sean Barney, a former aide to Gov. Jack Markell and the 2014 Democratic nominee for treasurer, announced Tuesday he will seek the soon-to-be vacant seat.

The current congressman, John Carney, a Democrat, is giving up the seat to run for governor in 2016.

This marks just the second time the U.S. House seat will be open since 1992.

Mr. Barney, 40, joins four other Democrats and two Republicans who have filed or announced their candidacies.

In a left-leaning state, the primary election — currently featuring four big names who are connected in party circles — likely could be the toughest test Democratic candidates face.

State Sen. Bryan Townsend, D-Newark, Rep. Bryon Short, D-Arden, and former Delaware Labor Secretary Lisa Blunt Rochester have filed, while Mike Miller has announced his intention to run.

On the Republican side, former Wyoming Mayor Hans Reigle and 2014 GOP nominee Rose Izzo have filed to run, as well.

Sean Barney

Mr. Barney is president of the local venture capital firm InfoVest and previously worked for both Sen. Tom Carper, D-Del., and Gov. Markell.

His Tuesday announcement focused heavily on foreign policy issues centered on the Syrian civil war and military intervention against the Islamic State. A former Marine who was wounded in Fallujah, Iraq, Mr. Barney believes the United States needs a firmer strategy.

“Marines have a code of honor. We leave no one behind,” he wrote. “Yet in this country and this state that I love, we are leaving people behind in droves. We are better than that. It is time that we start acting like our better selves and building a Delaware and an America again that embraces opportunity.”

Sending more troops into the Middle East would be a “serious mistake” in the absence of a long-term plan to stabilize the region, he said in an interview.

President George W. Bush’s administration was not prepared for much beyond an initial invasion of Iraq, Mr. Barney alleges — something he said he learned firsthand as a Marine.

“There wasn’t a plan for the day after,” he said.

He also supports taking in 100,000 Syrian refugees, far beyond the 10,000 the White House has called for.

The recent debate of whether to welcome or turn away those supposedly fleeing the violence in Syria has mostly been shaped by political leanings. Republicans, including governors and legislators, have pushed for the United States to bar entry out of concern that terrorists would enter the country. Democrats claim existing security measures are sufficient.

The belief that the current screening process is strong enough is one Mr. Barney subscribes to.

Unlike Europe, refugees are not simply crossing the borders en masse but have to go through a two-year security check, he insisted.

Preventing refugees from leaving the war zones and crowded camps in the Middle East is weakening regional allies and serving as “ground zero” for Islamic State recruitment, Mr. Barney said.

He said he intends to focus much of his campaign on economic issues. as well.

“Workers are helping grow the pie and they’re not seeing the benefits,” he said.

Mr. Barney ran for state treasurer last year and became the Democratic nominee after incumbent Chip Flowers dropped out of the race. Republican Ken Simpler won the office, 54-44, in a year that saw record-low turnout.

In total, 36 percent of registered Delaware voters in cast a ballot, which Mr. Barney says factored into his defeat.

How many people vote in 2016 will play a large role in determining the winner, although with both the president and the governor’s offices on the ballot, turnout is expected to be much higher.

With five Democrats running for Congress, the victor of the primary easily could claim less than 50 percent of the overall vote, winning with a plurality.

Mr. Barney is backed by VoteVets, a self-described “progressive” political action committee supporting veterans of the wars in Iraq and Afghanistan.

Mr. Barney’s announcement touts an array of liberal positions on issues such as his call to “pass legislation raising the minimum wage, ending mandatory consecutive sentencing, creating universal background checks for gun sales, passing marriage equality and establishing civil rights protections for transgendered Delawareans.”

Rep. Short has received endorsements from colleagues in the General Assembly, while Sen. Townsend has earned support from educators and community leaders, as well as several legislators.

Mr. Barney pointed to his background as the only candidate who has run statewide, believing it sets him apart from the pack.

The primary election is Sept. 13.