With all 25 Senate seats up for grabs, and Republicans vying in many of them, there’s a better chance now than in most years to reconstitute the party mix. Add the fact that reapportionment is forcing some veteran incumbents to run against one another, and voters have some tough choices come Aug. 11 — or as early as this week, if voting by mail-in absentee ballot.

Five incumbents are running unopposed so are assured returns: Senate President Shan Tsutsui (D-Wailuku-Kahului), Suzanne Chun-Oakland (D-Nuuanu-Liliha), Donna Mercado Kim (D-Moanalua-Halawa), Glenn Wakai (D-Kalihi-Salt Lake) and Jill Tokuda (D-Kailua-Kaneohe).

But on Oahu, five districts have contested Democratic primaries, some pitting veteran politicians against each other. Here are our endorsements in those races:

» 11th District (Makiki-Tantalus-Manoa): The most exciting Oahu Senate race is here, where two longtime incumbents square off due to reapportionment — Carol Fuku­naga versus Brian Tani­gu­chi. The two have been on similar political tracks for some 30 years: Fukunaga, an attorney, first entered the state House in 1978 and has been a senator since 1992; attorney Taniguchi, now a Hawaii Government Employees Association employee, entered the House in 1980, then the Senate in 1994. Their voting aesthetics, too, have been similar — in fact, they co-chaired the Senate money committee at one point — and their longevity in elective office speaks well of their neighborhood ties and involvement. If not for redistricting, each would likely have continued in office until retirement. But there can be only one winner, and based on a higher visibility and engagement in this redrawn community, Taniguchi should prevail.

» 17th District (Waipahu-Pearl City): First elected to the Senate in 2004, incumbent Clarence Nishi­hara now faces a challenge from former state Rep. Alex Sonson — again. Since a temporary suspension of his private law practice in 2004 due to an ethical lapse, Sonson, who served as a state representative from 2002 to 2008, has rebounded with college teaching stints and community activism in Waipahu senior and youth centers. Nishihara, a retired career educator, can be viewed as a dependable Democratic vote, and has not been one to rock the boat. Both candidates appear immersed in their community, but Nishihara’s incumbency inroads give him the leg up on his constituents’ behalf. The victor here wins the seat outright.

» 18th District (Mililani-Waikele-Kunia): Incumbent Michelle Kidani faces former state Rep. Michael Magaoay in this race. Since her 2008 election, Kidani has risen to the occasion as a lawmaker as well as continued community leader. She served as vice chairwoman in the Senate’s powerful money committee last session, and has kept strong, positive connections with her constituency.

Electrical consultant Magaoay also has a healthy record of civic engagement, and was a state representative for 10 years, so his candidacy is quite viable. On balance, though, Kidani’s solid showing in her freshman term holds promise and we’re curious to see if she develops into a statewide leader.

» 19th District (Ewa Beach-Iroquois Point): In a West Oahu district that could see much more growth in the near future, incumbent Will Espero has the energy and inclination to serve his constituency well. A full-time legislator since 2000, Espero has been active in his community’s events and growth, and should be allowed the continued opportunity to help shepherd infrastructure needs through. He is running against Roger Lacuesta, who has kept a comparatively low profile. With no Republican foe, the winner here gets this office.

» 25th District (Hawaii Kai-Waimanalo-Kailua): This district race made front-page news when Laura Thielen battled the Hawaii Democratic Party for refusing to let her run as a Democrat; she had once served as state land director under Republican Gov. Linda Lingle. Her protest sparked dissension within the Democratic Party, which district incumbent Pohai Ryan, elected two years ago, has long and loyally served. Also running in the Democratic primary is Levani Lipton, a political neophyte and Ananda Foundation executive director.

But it’s Thielen’s experience in land matters that would prove valuable as the tension between agriculture and development ratchets, and she is promising transparency in environmental protections. In a Legislature with too many members who go along to get along, Thielen offers fresh potential for healthy discussion, even dissent. We’d like to see her advance. The winner moves on to face Fred Hemmings, the longtime GOP legislator who vacated this seat a couple of years ago.