CHICAGO >> U.S. Rep. Tammy Duckworth won the Democratic nomination for U.S. Senate today, making official a matchup with Republican U.S. Sen. Mark Kirk that’s been informally underway for months.

The already-contentious race is expected to be one of the costliest and hardest fought of the November election. Democrats see Kirk’s seat in this typically blue state as one of the party’s best chances to unseat an incumbent as they try to regain control of the U.S. Senate.

Duckworth, an Iraq War veteran who attended McKinley High School and the University of Hawaii in Honolulu, used big advantages in fundraising and name recognition to defeat former Chicago Urban League CEO Andrea Zopp and Napoleon Harris, a state senator and former NFL player from Harvey who owns several suburban Chicago pizza restaurants.

Zopp, of Chicago, went after Duckworth particularly hard in recent weeks, saying she had little to show for two terms in Congress and criticizing her for not participating in more than one televised debate. Both she and Harris, who are African American, also argued they would do more to reduce gun violence, improve the criminal justice system and bring jobs to struggling neighborhoods.

Duckworth kept her focus squarely on Kirk, who is seeking his second Senate term after five terms representing Chicago’s northern suburbs in the U.S. House.

Kirk, of Highland Park, defeated Oswego businessman James Marter on Tuesday after a primary in which the little-known candidate presented himself as the more conservative option.

“I really am honored to have the backing of my fellow Republicans from this state to carry the banner and make sure we beat Tammy Duckworth in November,” Kirk said Tuesday night.

He said he expects the race to focus heavily on national security issues such as how to contain the Islamic State group and the nuclear deal with Iran.

Duckworth has said she supports allowing 200,000 refugees — including 100,000 from Syria — into the U.S., a number much higher than President Barack Obama has supported. Kirk’s campaign has called that position “naive.” He wants to prohibit any refugees from entering the country until federal authorities can guarantee they do not pose a threat.

Kirk also was a vocal critic of Obama’s nuclear deal with Iran, which Duckworth supported.

Both Kirk and Duckworth have made compelling personal stories part of their campaign. Kirk suffered a near-fatal stroke in 2012, but returned to work in Washington a year later following intense rehabilitation. Duckworth, who grew up poor and attended college with help from federal grants, lost both legs after the helicopter she was co-piloting was shot down in Iraq.