The California presidential primary is getting all the attention today, but there are several House primaries that could go a long way to determining if the GOP can hold on to its majroity.

In North Carolina's Second District, Rep. Renee Ellmers – the only House candidate endorsed by Donald Trump – is facing off against another incumbent, Rep. George Holding.  The two members were thrown into the same district as a result of a redrawn district map in North Carolina.

That race will be close, as will the contests in North Carolina 3, California 24, and Iowa 1.

The Hill:

Ellmers, a Tea Party favorite when she was elected in 2010, now faces a conservative backlash.

Conservative groups such as Americans for Prosperity, which is backed by GOP donors Charles and David Koch, and the Club for Growth are focusing their efforts on defeating her. To make matters worse for Ellmers, half of her new district was once Holding territory.

Ellmers came under fire from anti-abortion and conservative groups last year after she worked to temporarily halt a bill that banned abortion after 20 weeks. Multiple female House GOP lawmakers worried the original legislation treated rape victims unfairly.

Ellmers was among the first GOP lawmakers to endorse Donald Trump for president in an effort to boost her conservative bona fides ahead of the primary.

Yet that may not be enough to keep her in Congress. The most recent Federal Election Commission records showed that Holding had raised about $1.6 million through mid-May, compared to Ellmers’ $1.3 million.

North Carolina’s 3rd District

Rep. Walter Jones faces a rematch against his 2014 Republican primary foe, former George W. Bush administration official Taylor Griffin.

While Jones enjoys widespread name recognition from serving in the House for more than 20 years, this could be his toughest primary challenge yet.

Jones has developed a reputation for frequently bucking his party on issues ranging from financial regulation, campaign finance reform, national security and federal spending. Jones argues it shows he isn’t beholden to GOP leaders, but Griffin argues that Jones’s maverick ways have left him siding with Democrats too much of the time.

Griffin has seized on Jones’s votes in recent years against the annual defense authorization bill. Jones’s district includes multiple military bases, such as Camp Lejeune.

Perhaps sensing a vulnerability, Jones last month voted for this year’s version of the defense policy measure. 

[...]

California’s 24th District

The race to succeed retiring Rep. Lois Capps in the Democrat’s Santa Barbara-area district is unpredictable.

GOP state Assemblyman Katcho Achadjian and Santa Barbara County Supervisor Salud Carbajal, who’s backed by the Democratic Congressional Campaign Committee, are expected to advance to the general election.

Republican rancher Justin Fareed and Santa Barbara Mayor Helene Schneider, a Democrat, are looking to pull off upsets.

Capps, who has served in the House since 1998, narrowly defeated her Republican general election challenger by a mere 4 points in 2014.

Republicans are hoping that the lack of a Democratic incumbent will heighten their chances of flipping the seat in their favor.

Still, nonpartisan political prognosticators such as the Cook Political Report and the University of Virginia’s Center for Politics rate the district as “likely” remaining in Democratic control.

[...]

Iowa’s 1st District

Freshman Rep. Rod Blum has been one of Democrats’ top targets since winning the seat previously under the party’s control in 2014.

Blum, a member of the conservative House Freedom Caucus, recently created national headlines after declaring that Washington, D.C., “needs a recession.”

Cedar Rapids City Councilwoman Monica Vernon, who’s backed by the Democratic Congressional Campaign Committee, is considered the favorite to win Tuesday’s Democratic primary to face Blum in November.

Yet she will have to edge out former Iowa House Speaker Pat Murphy, who lost to Blum two years ago amid the nationwide Republican wave.

Vernon is winning the money race, raking in $1.3 million, compared to $211,000 for Murphy.

Democrats see the race as one of the most critical to win in order to increase their House ranks. Political prognosticators rate the district as a “toss up.”

On election night, the results of these races will probably show a trend in which party is doing well and which isn't.  But with so many unpredictable results so far, it's hazardous to state anything conclusive about what will happen in November.