The matchup for an open Senate seat in Iowa is apparently set, thanks to Rep. Tom Latham's (R-Iowa) decision Wednesday not to run.

Rep. Steve King (R-Iowa) is now a heavy favorite to win the GOP nomination, assuming he runs (which is a good bet and looking stronger). In the general election, he would almost definitely be pitted against Rep. Bruce Braley (D-Iowa), who has a near-lock on the Democratic nomination.

Rep. Steve King (R-Iowa) (Chip Somodevilla/Getty Images)

But King still makes many Republicans nervous. Indeed, when Karl Rove recently helped launch a new group seeking more electable Senate candidates, it used King as Exhibit No. 1 of the kind of nominee the group would like to avoid.

Latham was clearly the GOP establishment's best hope to avoid nominating King, and Iowa Republicans say it's unlikely anyone would give King a serious challenge now.

"Assuming Congressman King wants to run, he would be the closest thing you could get to a lock on the nomination," said one Iowa GOP strategist, granted anonymity to discuss the race candidly. "I can't see anyone in Iowa who could beat him in a primary."

Another Iowa Republican said the Senate GOP made a strong pitch to recruit Latham, and that his decision not to run was not for a lack of trying on their part.

"The NRSC certainly made a good-faith and heavy-duty case that he is the right guy and they would support him," the Republican said. "I think he is the major guy. Clearly, after Latham, everybody else was Plan B."

Alternatives that have been mentioned include Lt. Gov. Kim Reynolds, state Agriculture Secretary Bill Northey and state Sen. Brad Zaun. But any of them would begin a campaign against King with little hope of beating him.

(Some are also suggesting that Latham hasn't totally closed the door on the 2014 Senate race, noting that he said only that he didn't want to spend two years campaigning. Latham's office has yet to clarify his plans, but giving King such a head start would make Latham's task significantly harder than it already was.)

The question for Republican establishment groups now is whether they are concerned enough about King's potential nomination that they would wage an expensive and uphill primary battle against him — assuming they can find a warm body to serve as the candidate. In doing so, they would risk bloodying their eventual nominee and draining resources.

In the end, the best option may be to just embrace King who, for all his potential liabilities and his very conservative record, has put together a pretty solid electoral record. He is just coming off an eight-point victory over the well-funded campaign of former state first lady Christie Vilsack (D) in a district that included plenty of new territory. King's performance in the district matched Mitt Romney's very closely.

King has also recently suggested he might blaze a more moderate path in his Senate campaign. When a bipartisan group of senators came forward with their new ideas for comprehensive immigration reform, King (known to many as a top immigration hard-liner) praised the ideas.

But you don't have to do too many Google searches to figure out why King gives his party heartburn. He led the charge last year in suggesting that former Hillary Clinton aide Huma Abedin had ties to the Muslim Brotherhood, and suggested in 2008 that Obama's election would lead radical Islamists to declare victory in the war on terror. (This is just a brief sampling.)

When Republicans have run into problems with ultra-conservative candidates like King, it's when they have been unable or unwilling to tailor their message to the race at hand. They can't get over their ideology and wind up getting tripped up in areas where most other candidates don't even venture.

King's past suggests he's susceptible to that, which is why the party is looking for other options.

But he might be their best/only option at this point.

Update at 5:57 p.m.: King says in a statement: "A potential Senate race remains an analytical decision first and then one that requires deep conviction. Such a decision includes, of course, my family and the best interests of Iowans and Americans. It is too big a decision to be rushed."

Sean Sullivan contributed to this post.