BOSTON — Republicans trying to crack the state’s all-Democratic congressional delegation are pinning their hopes on a pair of competitive House races: one in the 6th District north of Boston and the other in the 9th District that includes parts of southeastern Massachusetts and Cape Cod.

By far the most attention – and outside money – has been focused on the 6th, where former GOP state Sen. Richard Tisei is vying against Seth Moulton, an Iraq War veteran who upset incumbent U.S. Rep. John Tierney in the Democratic primary.

Republican John Chapman, meanwhile, is trying to topple two-term Democratic incumbent William Keating in the 9th.

The GOP won two U.S. House races in Massachusetts in 1994 and hasn’t won any since. The party is fielding candidates in only three of the state’s nine congressional districts.

Tisei narrowly lost to Tierney in 2012 and was anticipating a rematch with the veteran Democrat before Moulton turned the race upside-down by ousting Tierney in the primary.

Moulton says his experience as a U.S. Marine in Iraq would be valuable in helping deal with the threat posed by the Islamic State group and other militants. In a recent ad, he questioned Tisei’s commitment to veterans, citing his 2009 vote against a state budget that included funding for veterans.

It led to one of the campaign’s most bitter exchanges.

“There was never a vote taken to cut veterans benefits,” Tisei said. “I voted against the entire budget that year because there was a billion-dollar tax increase.”

The race has drawn national attention, with more than $3.4 million in outside expenditures for both candidates through Monday. By contrast, super PACs have yet to throw any money at the 9th District race.

Chapman, who worked in the Ronald Reagan White House after college and later served under former Republican Gov. Mitt Romney, is making his first bid for elected office. Keating cites his own record of achievement including his service on the House Homeland Security committee.

The candidates have also clashed over flood insurance, a sensitive issue in the district, with Chapman faulting Keating for voting for a 2012 spending bill that led to sharp hikes in insurance premiums.

“He could have raised his hand and said, ‘Wait a minute, this is impacting my district too much,'” Chapman said.

But Keating said a vote against the broader bill would have had severe consequences for the district.

“The criticism would mean no flood insurance. It would mean doubling the (student loan) rate. It would mean not going ahead on South Coast Rail and other transportation projects,” said Keating, noting he later challenged the methodology for developing new flood zone maps and supported a bill to freeze insurance rates.

In the state’s only other contested House race, Republican Roseann Wofford is challenging incumbent Democrat Niki Tsongas in the 3rd District.

Christopher Stockwell, an independent, is also on the ballot in the 6th District.