PLEASANTON (KPIX 5) — A race for a California state assembly seat is heating up. It’s the last seat held by a Bay Area Republican.

Incumbent Republican Catharine Baker and Democrat Cheryl Cook-Kallio are locked in an expensive battle for the Tri-Valley seat.

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Right now, Assembly District 16 includes Alameda County and Contra Costa County. It is represented by Assemblywoman Catharine Baker — a Republican in a district full of Democrats.

But with Democrats just one vote shy of a supermajority in the Assembly, this seat is a big, juicy target.

Baker said, “About 40% are Democrats but over 50 or 60% are not Democrats in the district so it’s a swing district and it’ll always be challenging.”

And it’s a challenge that Democrats are taking head on because there is more than this district at stake.

Matt Shupe is a Republican strategist and he says it’s all about stopping Democrats from becoming too powerful in Sacramento. With Baker in office, it helps keep Democrats from a needed 2/3 majority to pass tax increases and change election laws.

“Republicans have been wanting to prevent the Democrats from getting a supermajority so they could continue to fight taxes,” Shupe said.

According to Shupe, “…the Democrats really want to break that hold that they have and be able to completely run through their agenda.”

Baker explains how she sees it.

“The most important part is when one party has so much power they don’t have to talk to the other side that’s bad whether it’s Republicans in Congress or Democrat…”

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Cook-Kallio said in a statement to KPIX 5 that “This race is so important because the people in this district want change.”

Both parties have poured massive amounts of money into this race. Baker has raised more than $3 million, about $1 million of that is from the GOP.

Cook-Kallio has raised nearly $2.5 million and about $1.5 million is from the California Democratic party.

It is by far the most expensive assembly race this election.

And all that money can buy some seriously negative ads.

One thing you won’t see?

Either side bringing up Donald Trump.

Baker said, “I’m voting for Condoleezza Rice in this race. I think she would’ve made a great president. That’s a Republican I’m proud to vote for and I’ve been pretty public about that for the last six months.”

In addition to the millions raised by the campaigns, more than $1 million has been spent by independent expenditure committees – almost all of it in favor of Baker and against Cook-Kallio.

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But it seems like every day a new large contribution is reported, so the spending in this race is far from over.